For what it's worth the SR40 Click connect is really no bigger than the onBoard. And it's the exact same carrier as the SR35 LX click connect - just with different labels stating different weight and height limits.
Honestly, for someone who rides in cabs frequently - euro belt routing is not a bonus, it's a hassle. I can install an infant seat without the base in under 20 seconds using my technique but having to install with euro routing takes longer and that would get old fast. Plus, cabbies are not exactly known for their patience. Of course you could opt NOT to install using the euro routing and just do standard belt routing instead but personally I wouldn't try to sell someone in this situation on a seat with euro routing.
I would suggest on Onboard 35 (basic model - no reason to pay extra for a premium base that won't get used) or maybe a Shuttle but honestly, I'm not a big fan of the Shuttle. It's difficult to tighten the harness on the lower height settings.
Here's a video showing my baseless install technique:
Video Link: http://www.youtube.com/watch?v=2FRR1koMF2c