Have you tried custom g-code to select each extruder individually and set its e-steps per mm? Don't know if it will work, but it looks like the only way to do it through Slic3r.
T0
M92 E720
T1
M92 E700
There may be something you can do with the tool-change custom g-code and placeholders, but I can't see anything obvious (or even not obvious). Can't seem to find a list of placeholders anywhere.
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