Yes - the mount is missing in a slew by a repeatable amount for a given mount and a given payload. So compensating for it should work just fine. Precisely because the error is in the slew and not in the sync. Any error of any kind that is repeatable in a slew would be corrected by offsetting it - unless the mount is knowingly conspiring against it.
There is always a chance the ascom driver is doing something weird in all this - and I hope not. If I aim for a given position and it lands consistently off by 1 degree - then I aim for a position away from it by one degree - it should offset by one degree per my request. If it doesn't - it would be hard to explain what is going on since we know from the logs it is very consistent in hitting a given target - with a precise offset.
But I don't think ascom is involved at all here. Again - I sure hope not.
Frank