I have played BRD for quite some time as well, but took a long break after WotG came out (at the the expansion was only partly rolled out) and only recently rejoined FFXI. While I (think I) understand your request, it would seem that for the situation described, simply forming all-mage and all-DD parties for the prebuffs would essentially solve your problem. There is no reason to pianissimo. Since you as the bard are already party hopping, why not make your job simpler by having the others also party hop. Using this method, there is no need to overwrite melee songs with mage songs (and/or vice versa) -- this could possibly give you time to do even more songs for other groups. This should be fine for pre-buffs right before starting a battle. You sing your songs, no overwrites except for songs 3 and 4, and everyone reforms into their proper parties. Macro out the invites if you have to to save time.
This would not work, however, if your fights are fairly drawn out and troubador/nightingale needed to be used multiple times. However, I wonder if this may still be a non-issue mid-battle since by then the parties are usually separated enough that you would not need to overwrite a melee song with a mage song (i.e. your healers are generally not standing side by side with your melee).