Are there any additonal drawbacks to permanently attaching the keel to the hull, other than transportation and tuning? Thinking about doing it to my USOM since it will be more of a Sunday sailor than a regatta rat.

One difference between the US1M and the IOM rules, is that there is no requirement to weigh the fin/balast, so it need not be removable.

The only drawback I can see is that it would be more work to remove the fin if it were damaged and needed replacement.

I can see an advantage. You could eliminate the keelbox and save some weight.