I'd advise against making changes while a
dcommit is in progress, although that may work if you're always pushing out your
HEAD commit and it goes cleanly.
HEAD is being
dcommitd, the process ends with doing
git reset --mixed against the new commit which reflects the current state of the svn repository. If you'd instead push some older commit, the process would end by attempting to rebase your current branch on top of the new commit that was fetched from svn; this would abort if there are uncommitted changes.
You're also likely to have problems if
dcommit would run into a conflict while pushing the changes to svn, which you would then need to resolve manually. Having unrelated changes in the working tree is likely to mess this up.
You could create a separate clone of your
git svn repository, do all your actual work there, and use the
git svn repository only for interacting with the svn repository. Then it would of course be safe to make any changes in your working repository while the other one was processing a
dcommit operation. But you would, of course, need to be pushing and/or pulling commits between those repositories.