So, I kinda did what I said in the end of my post.
Using a couple different capes and the magical
Caller's Sash (the most OP matk item in the game), I was able to get some numbers.
On Meteor Strike, 0 TP, 5/5 merits:
Hagondes Sabots (MATK+45)
BP dmg+1 = +55 dmg
MATK+1 = +28.5 dmg
Helios Boots (BP Dmg+7, MATK+26)
BP dmg+1 = +51 dmg
MATK+1 = +29.5 dmg
As you can see, with more MATK, BP dmg becomes more useful, and with more BP dmg, MATK becomes more useful (this was kinda obvious, but hey look numbers). With the Hagondes Sabots, 1 BP dmg is roughly equal to 1.93 MATK (almost double), while in the other case, 1 BP dmg is roughly equal to 1.73 MATK. The value of MATK can change drastically with increases to BP dmg, while the value of BP dmg will change with MATK.
Either way, with these numbers, I can guesstimate my damage gains off these stats a lot better, and say that BP dmg is worth roughly 1.5 ~ 2.0 MATK, which is a much better ballpark than I had before (none). Using this as a metric, KEEPING IN MIND that this is not 1000% accurate and that the weights change a bit with adjustments, I can make some gear decisions. Because I will still be leaning favorably one way or another on scaling, we should be able to get a pretty good idea unless the numbers come VERY close.
Also keep in mind that I am using Nirvana, which drastically affects my scaling. With a Balsam or Keraunos, the worth of BP damage goes way up, so if they're very close in value, the BP damage might pull ahead in those cases.
These tests for for Magical Blood Pact Rages, not Flaming Crush. Flaming Crush gets much more worth from BP damage.
Shomonjijoe+1 (MATK+30) vs Convoker Doublet (BP+12)?
Even if BP damage was at its most optimal (2 matk per point), it still can't get close to Shomonjijoe, with a difference of MATK+6. Even at a minimum unity ranking (which never happens), Shomonjijoe still barely pulls ahead by 1 MATK. If you're using Balsam or Keraunos, this one might be tough! BP damage greatly increases the worth of Shomonjijoe.
Verdict:
Shomonjijoe +1
Hagondes Sabots (MATK+45) vs Helios (BP+7, MATK+26)?
Same thing here. Even if BP damage was most optimal at 2 MATK per point, that would still only leave the Helios Boots at MATK+40, which is not enough to scratch the Hagondes at +45. Even if the MATK on the Helios was capped out at +30, it would still lose, just barely, to the Hagondes, and the Hagondes are not capped out either (MATK+20 instead of MATK+25). Helios is no good for Magical Pacts in this case, however, for Flaming Crush, it's still worth it! Trading what amounts to be roughly MATK+5 for BP+7 is totally worthwhile, and then still some bonus crit rate. If you're using Balsam or Keraunos, the worth of the BP damage from the Helios will go up here; not sure if enough though!
Verdict:
Hagondes Sabots
Bonus Round:
Conveyance Cape (BP dmg+5) vs Argochampsa Mantle (MATK+12)?
Again, Argochampsa wins on damage. Even if BP damage was at optimal value, Argochampsa still pulls ahead by 2 points. However, it's important to keep in mind the Conveyance Cape can have anywhere from +9~13 summoning skill on top of it, which roughly equals that much accuracy. You'd be trading +2~4 MATK for +9~13 accuracy. Tough, this one is gonna be up to you. If you're using Balsam/Keraunos, again, Conveyance Cape might pull ahead.
As you can see, in a lot of these, the winner is quite clear, but they can be kinda close sometimes. I'm only testing with Nirvana, because that is what's relevant to me at the moment. I might go back and test with others if I feel like it, but either way, it's going to be very difficult for me to give up using Nirvana even if the damage is slightly higher with Keraunos/Balsam because of Nirvana's accuracy gains.
EDIT: Fine, I wasn't going to test Balsam, but curiousity!
Balsam Staff
BP dmg+1 = +73 dmg
MATK+1 = +20.5 dmg
This puts BP dmg equal to roughly 3.56 MATK per point. Ouch! This will definitely affect your gear decisions above. Convoker's Doublet and Conveyance Cape pull ahead, becoming better than their other options. This also serves to make Helios Jacket even stronger, pushing it ahead of the Convoker's Doublet. However, in the case of Hagondes Sabots vs Helios Boots, it comes VERY close, with Helios being like +1~2 worth of MATK ahead. If you stick BP dmg on Keraunos, Nirvana stands no chance in any of these cases for damage, but Nirvana still has that massive accuracy lead.