That may be, but the fact is that mortgagor is used 10 times more than mortgager, so whether it's an acceptable spelling or not, its value is severely cut because of that.
Also, what exactly would you develop on the domain? A site for lenders? MyMortgager wouldn't really be a place for borrowers since they ARE the mortgagor. Also, another thing to note is that borrower is by far the more used term anyways. I work at a national appraisal firm and have been for 2 and a half years, and have heard "mortgagor" maybe 10 times while we say "borrower" or "homeowner" in dealing with almost every single order that comes in the door. The OVT search amount is low for mortgagor (435) and even less for mortgager (60). Borrower isn't even searched a WHOLE lot (1337), but is sure used a whole lot more (24,100,000 Google) than mortgagor (1,210,000) or mortgager (138,000).