Nicolas, can you resolve source locations at SWANK level? E.g.
functions like SWANK/SBCL::SOURCE-FILE-SOURCE-LOCATION might be
appropriate for patching: you take the location that SBCL gives you,
but then redirect SWANK to other place. I guess this is not the only
place you have to patch, it would require patching private functions
and it might not work at all. But if that works for you please let me
know. I'm also maintaining a thing a bit similar to portacle and it
looks like I would suffer from the same issue. 

Also let me remind you that you can set up the path to SBCL sources by

sb-ext:set-sbcl-source-location

AFAIR locating sources of SBCL functions didn't work in portacle when
I saw it, and it was not very long ago.