Even if you do find a way to reduce the power through software, the PA will be much less efficient and still produce a good amount of heat at the less efficient power output level it wasn't designed for. If you are doing this to prevent PA failures and to drive an external amp, you would be much better off using a good attenuator with proper heat sinks and cooling to burn off some of the wattage to drive an external amp. In other words, you would be placing more burden on the PA running it at a much less efficient lower power than it was really meant to operate at. You could also put the PA in a situation that it oscillates, and again putting more burden on the PA than you may be desiring to have. The reason there is low power and high power version of these repeaters is because the PAs are of different designs to be able to operate with proper efficiency in the power range that are rated for.
Even if you have to roll up a wad of RG-174 and put it in an RF tight box with good RF connectors to make a somewhat attenuator, you would be better off than hex hacking to make a PA run lower at a much less efficient level than it was designed for. Forcing the PA to run outside of it's efficient range can ruin the PA just as bad in the long run.
Not trying to rain on your parade, but just trying to give some solid RF advice about PAs.
