I did some more comprehensive investigation into possible counters to the Beowulf dhc setup which cannot be blocked, and was going to make a long neatly organized list but I found so few that I'll just name them along with some things that seem like they might work, but do not.
First off, all raw tags can be reacted to during center stage and then blocked after recovering.
I did not consider level 3's, and I did not consider invincible supers into some good dhc, because while those are relevant I will assume that Squigly > Beowulf can at the worst be completely safe if they use whatever optimal counter they have, as Squigly has the luxury of being able to choose which super she wants to start with. It is also pretty important to keep in mind that the defender has no way of knowing this setup is coming, so it is basically any other reversal situation in that regard.
Here are the ones I found:
Defending Squigly can do level 2 DP, which opens up a whole series of possibilities of supers and counter-supers, but in the end I firmly believe that the defending Squigly can be completely safe at worst, given the assumptions mentioned above. I tested a bunch of stuff and if someone wants more details ask and I'll try to write something up. Defending Squigly can also cancel the DP into SBO on block if the setup was not done, which is pretty much safe, usually.
Finally, Filia can use Fenrir, which has 3f startup and will hit Squigly out of center stage startup since the setup has a 4f gap to accomodate for jump startup. This is obviously not safe unless you have a good dhc.
Since Big Band's SSJ has 2f startup it seemed like it would also be a counter to this, but he is so big that the setup can be adjusted for him to not have the jump startup gap, he will get hit by the anti-air grab regardless.
TL;DR Squigly with charge can avoid it, Filia can counter it. There are no passive or reactive ways to defend against it, and if the definition of unblockable isn't that which cannot be blocked I don't even know man'
Edit: One final thing, you can time a frame-perfect delayed jump so that you will be in jump startup during the center stage flash. If a robot with <=16ms reaction time plays Skullgirls, this might be useful to them.