Alright, for anyone who might stumble into this problem in the future, here's the answer.

I had used the command "openvpn --config /media/mmc1/file.ovpn" to open the config file on the external memory card. The same directory contains the .p12 certificate file. The result was "error opening file.p12". Turns out that for whatever reason, openvpn was unable to see the p12 file, requiring it to be specified at the command line with the config file, in this form:

openvpn --config /media/mmc1/file.ovpn --pkcs12 /media/mmc1/file.p12

This could simply be me missing something completely stupid, but the command above has solved it for me.