The site template I am using has a working modal but the form is stored in a JS file. I used javascript web parts to surround the login form but it was too imbedded to use that way.
I them tried wijmo modal which toggles a containing div based on its ID but there seems to be a conflict becuase the console lit up like a christmas tree and java is not my strong point.
I also looked at the Properties Tab > Javascript Command to try and get it to open as a modal dialogue but I could not get it to open a page