I had an idea that would make it possible to support attaching in the XS UI for the soft debugger.
Basically, we could define some directory (maybe in the user's home directory, or the temp directory - obviously permissions/security would be a concern) that would contain debugger enumeration files. Then, any Mono process started with debugger options including "server=y" would write a mono-soft-debugger-$PID file to this directory, containing the port om which the debugger was listening. It woudl also be possible to use a auto-assigned port (i.e. port 0) because the port could be determined by reading this file.
Then, MonoDevelop could list attachable processes simply by enumerating these files, and connect to the port described in the file.
It would also be nice to have a --attachable option for Mono that would start the soft debugger with port=0, server=y, suspend=n. Then one could set MONO_ENV_OPTIONS=--attachable and all processes would be attachable.