This is meant as a serious question!
* Code for pro-drums exists in RB3 so it might 'just' need conversion
* Pro-drum notation was included since RB1 (just not used) so I'm guessing the pro-drum notation for TB:RB is alredy there
* Since it would just be a patch for the existing game I assume it wouldn't require any new licensing
All of this leads me to believe that it could be just a time/money issue. The demand is probably not that high so I could fully understand the reluctance to spend the money.
BUT, depending on what dollar number we'd be talking about, maybe a kickstarter campaign could get us there! The pro-drummers out there might be willing to pay more than expected to get the feature and in any case the project would only go ahead once enough pledges are in. It should be possible to make the spend interesting for Harmonix. The kickstarter campaign should have some added benefits with regards to advertising and could work as a test case for maybe other things (pro-guitar/base?).