I imagine some others have already noticed this, but git svn commands can be added as "Scripts" that are available from the history context menu. For instance, for "git svn rebase": Go to Settings > tab Scripts, click Add, enter a Name and be sure to click "Add to revision grid context menu", enter "C:\Program Files\Git\bin\git.exe" for the Command, "svn rebase" for the Arguments, then click Save. Now, with a right-click on the history graph in gitextensions, the new command should show up down at the bottom with the entered Name. Choosing it will bring up the expected output window. Similar steps apply for "git svn dcommit --dry-run" and "git svn dcommit".
It's a small change, yes. And for the full glory of git you still must click on the little terminal icon on the gitextensions toolbar and use the command line. But repetitive tasks should be made as rapid and unobtrusive as possible to conserve the programmer's cognitive load (which is why running one's unit test suite should also be extremely easy and painless). Of course, even the tiniest enhancement to workflow adds up over many times thereafter. Just as a mostly-positive monthly cash flow is the key to long-term financial sustainability, a mostly-frictionless development flow allows programmers to expend their valuable time on stuff that matters, like design.