Add commandline options to open bookmarked repos (or prefill the import dialog or import it directly).

For example:

SourceTree.exe --open bookmarkname
--> launch SourceTree and open the bookmarked repo 'bookmarkname' if it exists

SourceTree.exe --open c:\path\to\repo
--> launch SourceTree and open the bookmarked repo which points to c:\path\to\repo
OR open the 'Add working copy' dialog with the path prefilled

SourceTree.exe --open bookmarkname=c:\path\to\repo
--> launch SourceTree and open the bookmarked repo which points to c:\path\to\repo
OR perform an 'Add working copy' operation and then open then display the repo

additionally, one could drop the following bash script next to SourceTree.exe to trigger it via "git srctree"
or
register a windows explorer folder action to trigger it from explorer

   #!/bin/bash
   REPO="${1-$(pwd)}"
   SourceTree.exe --open "$(cygpath -w $REPO)"