How would I properly develop a current rating for this system...?
Make sure your fuse is rated to break 1.05kVAC. I would suggest fusing the primary as well, since the transformers could fail, in fact it's more important than fusing the secondary. 

According to the Hammond Design Guide for Rectifier Use, the maximum DC current output for your configuration is about 0.62 of the AC rating, or about 188mA in your case. This is because the RMS current in the transformer winding (and thus the heating) will be higher than the DC current output. 

If you're talking about current available at the terminals as shown, you have to subtract the current through the 400K bleeder resistor, which is about 3.8mA, leaving you with ~184mA.