It's not great tapping into the brake circuit for stuff like h/ grips (although some dealers seem to do it!) especially if you need to fit a bigger fuse...
A relay will be fine or why not just tap (splice) into the aux from the factory fusebox, a dedicated fused, switched live.
Alternatively buy Hondas 'quartet harness' and tap in to the accessory plug behind the l/h fairing.
Phil