tbh I did forget about it so not yet had a proper play about, I did have more of a think about it and I don't think its going to be as easy as I first though, for the reasons in my post a few above.
Getting it to exit a scratch is straightforward, identifying the exact entry point is a little more difficult especially without the option of setting a signal on the entry odds (which would, of course, make it straightforward).
The best workaround would maybe be if you could identify the approx profit you would still have at your entry odds after the first offset is matched then use that as your exit P/L