I've tried to cut some "Pondtabbs" from "Plantabbs". They just crumble. I have used them whole. They work well, maybe too well... I took a chance since I was due to redo the aquarium anyway. The sagittaria subulata took over the tank to the point when there was no longer enough circulation and it started to die off massively. That's when I redid my tank. Oh yeah, and the pond ferts I've tried "melt" into a fine silt in little time (minutes to hours). So that's a consideration to take. I did not mind personally. It depends on your setup and if you like to move things around I suppose.
I would watch out of ammonia and urea. Besides that, I cannot see why pond ferts would not work. I find that ferts are ferts. You just need to do a little math and compare your stuff with the stuff that is known to be good in tanks. If the elements present are relatively the same, why not? Plants don't really care in my opinion. As long as the ferts are there and in quantities that the plants need.