My LBS tells me that the rear brake can't really acccomodate a 23mm rim
Hey man, congrats on the new bike. You can definitely run as wide as 25mm rims on a TM01 - I myself run that for both my training and race setup. I'm using SwissStop pads but also used some other off brand cork pads with no problem.
When I switched to SwissStop pads, I had to back the pad holders almost all the way against the calipers. You may know already but you do this by moving the spacers from one side of the calipers to the other. See pg 26 of the manual for what I mean. This is all I did to get my brakes to accommodate my 25mm rim.
If that fails, there are some people who remove the posts that regulates the stance of the brake calipers (pg 21, last illustration, part #4 "brake stoppers"). That will allow the calipers to move as wide as the brake pads will allow. I don't see any concerns about removing those stoppers so I don't hesitate to recommend it to you either. Really can't think of a down side.
All in all, I'm very surprised you're having problems with a 23mm. It's definitely resolvable. Oh and your LBS probably doesn't deserve your business. http://www.bmc-switzerland.com/fileadmin/files/documents/04_Support/Manuals/TM01/TM01_OwnersManual2012_EN.pdf