The 1.7 series is a ton better than 1.5. It's just generally cleaner and more useful. You can also use Game_Battler.bloodmage_applies?, I believe, to check to see if Bloodmage can be used in the current situation -- I should probably get that to work with out-of-battle spells, if it doesn't already. 7_7
Anyway, what you're suggesting should be simple. You'd just have to mod bloodmage_applies and the skill effect method. I'll see what I can do in a sec.
EDIT:
Working on 1.7.3 currently; and lol, it's a year since I released 1.5.9! Wow, I'm sooooo lazy
*BUMP*Alrighty, new version released. It's a bugfix release and a feature release, so please, check it out.
G_G, this should provide what you want, so try it out soon
Also, WOW at this script. It has pretty significant power in nothing but 65 lines if you remove all of the comments and blank lines 0_0