I didn't have to jump through the those hoops when I pulled this trick in both RTW and BI (I was using the Eras pack 1.5/1.6 version though). What I did in RTW was basically 'moving' everybody up. (Unlockable factions becoming playable from the get-go, unplayables becoming unlockables) DON'T try to make the Senate or the 'Slaves/Rebels' playable. It gets very cranky.

One neat trick I discovered doing this is that, if you have campaigns running and have already beaten some of the factions you make unlockable, the CPU automatically goes back and makes the unlockables playable.

A similar trick worked with BI, but I strongly recommend NOT unlocking the Romano-British and Slavs, as they do not techinally exist when the campaign starts. The Lombardi, Roxolani, Celts and Burgundi will work fine.

On a side note, do not attempt to change the campaign start (very tempting with regard to the Slavs in BI), as it will wreck the descr_strat script.