I don't think this idea is going to work. The mill is not particularly stable speed-wise at the low end which is not going to be kind to taps. I also don't believe the reverse is instantaneous.
The users who have wanted to install a reverse switch on their mill are using this in conjunction with a boring head run in reverse.
If you need to perform a significant amount of tapping operations you may want to consider purchasing a "self-reversing tapping head". The are large and cumbersome for the mill. Might be best to make the holes (or at least pilots on the mill) and then use a drill press to drill/tap the holes.
