Ooh! I like that idea.
Here's the Google Maps location:
http://goo.gl/maps/CG7LZ
If he says he'll go car-free, I bet he'd be required to promise that his tenants (or condo owners) sign agreements not to apply for permits. They did that across the street here in the South End at Atelier (although later the owners tried to sneak their way into get it overturned).
Regarding 15 N Beacon, as some will remember, that was an apartment building turned into condos (from rentals to sales) several years ago, when the market got hot. They sold them all off (I assume) but it took a bit of time - it's a big building, it's not in a prime location, and the units hadn't been upgraded (as far as I know). Resales now run in a wide range of $350-$490 per square foot (based on sales I can pull up on MLS for the past year). So, a 4th-floor one-bedroom with 615-square feet of space sold for $240,000 in 2012 (including two parking spaces).
Perhaps the developer could offer spaces for rent/sale at 15 N Beacon, but I don't know if they are offered to non-tenants/owners.
The retail at 15 N Beacon isn't great, from what I remember when I've gone by there. Either empty or kind of dumpy. MLS shows several spots being put of for sale for several hundred thousand dollars with no takers. Not sure if retail can be successful further down at the corner of N Beacon and Everett, nor am I sure the neighbors would want it.
Now, if he offered to take over the used car lot next door for parking or for more units, maybe they'd like that.