Bartec has an site online where you can lookup the sensor for a vehicle and get the Relearn Procedure. Out of curiousity I specified a 2016 R series BMW motorcycle. The site identifies the OEM sensors manufactured by Schrader that apply but the Relearn Procedure just says "For the Relearn procedure, please refer to the vehicle's Owner's manual." so not very informative.

Gateways on cars serve a bridging function between separate CAN-buses and/or the LIN-bus. Only the more recent BMW bikes have a LIN-bus. I don't know what year the LIN-bus was adopted by BMW for its motorcycles. I am fairly sure that prior to the intro of the LIN-bus there was but one CAN-bus on BMW motorcycles and therefore there was no need for a gateway. Our 2010 and 2011 bikes certainly don't have one.

I see no reason why you couldn't program a universal Schrader ez-sensor for either a pre-2022 BMW motorcycle or the 2009 Saturn Astra, enter its ID in the sidecar bike's RDC using a GS-911 and wake up the sensor to see if it shows up on the bike's instruments. You need not install the sensor on a wheel to do a test. In fact when replacing a sensor it's a good idea to do exactly that so that you know everything is ok before mounting the tire on the wheel. I did that to verify that the 2009 Saturn Astra programming works onthe Schrader 33900 ez-sensor.

As I said, more familiar with cars and only deal with things when I have to fix them. Even then I stumble around in the dark sometimes LOL I have a 2008 GS. Very rudimentary "Not sure if they had TPMS as an option in those days" and a 2018 GSA, somewhat complex and pretty sure it would have a network and a gateway of some sort. Sharing info from various sensors. Been in and out with the GS911 a few times sorting things. Some what similar to the cars I work on. Except I hate the lack of info I get with the GS911 and how they interpret the data. I would like to see more raw numbers. Can't speak to any of the other bikes never had to play with any of them 
 On the Sensors. Pretty sure the Ez sensor will be much the same for programming between each of the numbers. Just the shape is different between the car one and the bike one.  I was thinking I could program a car sensor and place it in the sidecar wheel instead of adapting the bike sensor. Now that I think of it, I might experiment tomorrow. I'll let you know. 

Ok, Humble pie.. as usual I shot my mouth off before really knowing.. Just shoot me Ha!  I tried to program one of our sensors today to a BMW Moto but the tool will not do it. I had looked at out tool before and the menus have all the bikes listed but going back in today to try and the program part is not there.  Tried all the years. Nada. Our Prema tire supplier was in today so we asked about the 33900 sensors. He can get them, two weeks, but has never sold any. No demand obviously. 
The tool does have the Saturn Astra and will program a sensor to that configuration. Didn't try.
I was hoping I had a fix for my sidecar friends with the fancy rigs running car wheels but not at the moment. 

Good of you to follow up. I had read elsewhere that someone with a similar tool found that it would not program for BMW motorcycles despite they're being listed in the menu. I am confident that, if you programmed a sensor as if it were to be installed in a 2009 Saturn Astra, it would be compatible with the BMW RDC. I have tried it myself. I am not entirely surprised by the lack of ready availability of the 33900 as it would only be used in a car application for certain custom wheels. The 33900 seems to be more readily available in the US where I picked up one at an O'Reilly's to try out. I ordered a couple more from a Canadian online supplier and they will be delivered tomorrow but that took a couple of weeks.

How would you deal with the three wheels of a sidecar rig as the BMW motorcycle RDC only supports two wheels at a time? Some years of BMW's will support two different wheels sets but only two wheels at a time.

As a post script to the PDF document in my original post.....

I picked up a 4-pack of the 25080 straight valve service kit at KAL Tire for $17.48.

BMW's price for ONE valve is $33.50 plus the nut which is sold separately at $12.50.

BMW's price is totally outrageous.

We weren't worried about 3 wheels on the sidecars just being able to not have the big warning  pop up on the TFT of the new bikes that LBS and Mike are building. So not really about a functional TPMS so much as getting the pesky warnings to stay our. If one wanted the TPMS I think Garmin supported it for three wheels. Not sure they still do.
I am going to pass on your info and what I discovered to Mike. He may be able to use the Car sensor programmed to the Saturn. Then get the GS911 to register them to the bike. We'll see.