Hi Adam,
The bug you get is not coming from OpenMEEG, but from Brainstorm. In the files you sent, we can see that the leadfield was correctly computed (openmeeg_gain_seeg.mat), but it looks like its size does not match what Brainstorm expects at line 492 in bst_openmeeg.
The leadfield matrix computed by openmeeg has 114 rows, but I can't check how many were expected based only on the OpenMEEG files you sent.
If you are comfortable with the Matlab environment, you could try understanding where is the error coming from by placing a breakpoint in bst_openmeeg at line 492 (click at the left of the line), then starting the BEM computation. When the debugger stops, you can explore the contents of the variables.
Otherwise, you could try sending me the files and I could do it for you. Right-click on the subject > file > duplicate subject, then delete all files that are not necessary to run this computation. Then right-click on the subject > File > Export subject, upload the .zip file somewhere, and send me the link as a private message.
Francois