The math works just about right for the weld size reduction, and it looks like you could simulate the A53 with A36 plate. I would have the Engineer sign off on the substitution. The thing I wonder about is economics, How many of these do you have to do? The WPS process will not be inexpensive, and the filler metal is pretty pricey also.
What is your plate and pipe wall thickness?
You have a 100KSI base plate, a .220 eff. throat 110KSI weld, which indicates that your pipe might be ~.500 and .700 36KSI material. I think that with the joint configuration under discussion, you will have a .3125 x weld length of pipe doing all of the work.
Lets say your pipe is 10" OD, then you have ~9.8in^2 of pipe that is effective. You will have a very similar amount on the base plate, ~10.12 in^2. Just for grins, you will have ~7.02 weld area (through throat).
Do the multiplication, and you will see that if the base plate is given a relative value of 1, the weld will be 76% as strong as the base, and the pipe will be 35% as strong.
If you used the full .75 fillet, you now miraculously have ~23.5" of weld area on the pipe and now could develop ~85% of the previous base strength, and if welded with 70KSI, the weld would develop ~120% of the previous base strength.
I would be shopping for a different base plate or a higher strength tube/pipe member or something
FYI these numbers are fast and dirty, so YMMV.