Right, I'm regularly programming GM PCMs. The issue is the P04 PCM OS'* (as far as I can determine) don't support a tunable parameter for setting the charge voltage. Mike is thinking that he might have seen it supported in HP Tuners for the P04 PCM and has kindly offered to have a look. Interestingly enough, I have seen it supported in HP Tuners in several of the newer GM PCMs but not the P04. One guy on the HP Tuners forum says that he thinks that it was supported in the O.*. but HPT did not map it. I did several table searches in the binary with various tools but I still couldn't find it. But, hopefully Mike is able to find it.
If it turns out that it'* not supported in the OS, there are PWM hardware modules available that plugs in series to the PCM PWM output and permits you to offset the existing PCM PWM output signal. I've seen mention in forum posts that indicate these modules work well.

As i pointed out onn buickforums, this is a misunderstanding of how the 2000-2005 lesabre charging system works. By extesion, 1986/7 lesabre-2005. The computer wire to the PCM is not for controlling the charging. The only PCM that may/should have this capability will be the northstar PCM for 04/05 DTS from what I can tell.