Mailplane now supports CC and BCC in mailto URLs. Plus, it is now much more standard compliant, for instance it supports the same TO, BCC, CC or BODY parameters multiple times. Here are a few examples:
mailto:email@example.com
mailto:?body=line1
line2
mailto:?to=email@example.com&subject=mailto uri scheme&body=line1
line2&cc=email@example.com&bcc=email@example.com