With the proviso that the following probably isn't in the spirit of what you actually intended, In a sense this is currently possible in that there is a
git command that executes git with the arguments that you provide. For example, you can add the following to the command palette if you want to view the reflog (Note: you need to click the icon in the info bar to the left of the current branch name in order to view the output) :
"caption": "Show Reflog",
The downside being that it's not truly arbitrary because you need to decide ahead of time what commands you want to execute, and you can't (currently) set up something interactive like selecting the branch to use in the command and so on.