I was able to work through web support to get just the base fare ($72.55) applied to my $75 e-cert for a recent LAX-LAS roundtrip. This happened just as the e-cert promotion was beginning, and it took a LOT of time to make it work out. I'm not sure they'd do this now that some of the bugs in the process have been ironed out.

I'd suggest that the OP just book a slightly higher fare basis (i.e. T instead of S) in one direction - that should be about $10 more and should allow the use of the e-cert. One way to do this is to hold a reservation on-line that included a higher fare (you can probably just specify refundable) that recognized your e-cert, then call UAL and have them make changes to fare buckets that still allow for a base fare of more than $75. Then you can go back and purchase the ticket on-line, so there's no need to pay a phone charge, you get 500 miles for the on-line booking, and you can be sure that your e-cert gets used.

Good luck!