Since we're still figuring out the problems/quirks with each type of upgrade, I'm going to try and keep them separate for now. Each section will have relevant links with more detail and I'm just trying to keep the highlights here.
Note: this is unlike other customs - you only use C3 CON Tools for encrypting midi files, and the rest is manual - no "merge songs" or any of that. Creation/merging of your upgrades.dta and any songs.dta updates are done manually via a text editor or similar.
Pro Guitar/Bass upgrades:
Since Pro Guitar/Bass was always designed as an add-on DLC component, it's simple to add a Pro Guitar/Bass upgrade to an existing song.
Extract all the Pro-Guitar/Pro-Bass file "*_plus.mid" to a folder on your computer, run them one by one through C3ConTools->PS3 Converter->Encryption Options->Encrypt Replacement Midi File. Recent versions allow you to select more than one file and convert them as a batch if they're all in the same directory.
This should give you PS3 encoded "*_plus.mid.edat" for each song you want upgraded.
Add all the upgrades.dta of all the songs you want into 1 upgrades.dta, which can be done manually using some text editor.
FTP into your ps3, go to the regular HMX0756 folder, and create a folder named "songs_upgrades", go into that folder and upload all the converted "*_plus.mid.edat" files and the one merged upgrades.dta file you made.
Voice Harmony upgrades work, but the process is not as simple. For the most part, the process above is exactly the same, with a little addition, you have to go to the original songs/pack folder on your ps3, and change the songs.dta entry to match the one provided with the upgrade in the RB:HP package.
Ex: To enable Metallica - Blackened Harmony, follow the process above for the updates.dta and encrypting the midi file, uploading them to the same folder as above (HMX0756/songs_upgrades/).
Just doing that won't work, you have to browse on your PS3 and go to the "/dev_hdd0/game/BLUS30050/USRDIR/ccf0001/songs" (this is the metallica pack folder, would be different for a single song, or any other songs for that matters), and download the songs.dta file to your computer, and replace the info regarding the song "Blackened" with the one provided in the RB:HP package.
This is necessary so that RB picks up that the song will have harmonies, but if all you want are harmonies, this method seems to be fairly reliable.
Note: Harmonies upgrades appear to be included in all other RBHP upgrade mid files, but the harmonies won't work without touching the original songs.dta file too. Not sure yet whether NOT modifying the original songs.dta will cause play errors on other pro upgrades.
Upgrade path is similar to with harmonies - original songs.dta must be updated so the game knows that a keys part exists.
Pro Drums Upgrades:
Seems similar to harmonies + keys - requires modifying the original songs.dta file.
Multiple upgrades to the same file are tricky, since you need a merged _plus.mid file that contains only the instruments you want to upgrade and no others (or everything) and a matching original songs.dta file. But since it appears that some of the RBHP upgrades include multiple upgrade tracks, it might be as simple as grabbing the one that has everything you want.
General RBHP Notes and Info: