The same folk offer a sleeve type that can be lock tighted in place! I have never seen how thick the B4 pan is! But on a GSXR1100/B1200 eng i have installed the helicoil insert with pan still on the eng with no probs to my knowledge! This uses the same plug, Tap it and you will have to use some other plug. SAE/JIC/PIPE, tapered! YUK JMOP!
The part where the thread is, is about 3-4mm (approx 5/32") high (IIRC),.. it's not that big. We played with the idea of helicoiling it as well, but in the end, it turned out that the channels would be interferred with, or else, not enough threads would be inside to make us confident with the repair.
Do you know, since you didn't take the pan out to heli-coil it, that the last of the oil is not sitting at that coil and unable to escape? (That pan may also be different)
I believe that suzuki sells an over sized drain plug. (Or a good parts person should be able to see which bikes have a larger one than the original, i.e. a b600 drain plug, etc.) If they don't, you can get a metric one from many of the auto parts stores. No tapered or SAE etc bolts required.