Hmmm. I run with oscam 8024 (and got the same with 7940). I modified oscam.dvbapi as described (pmt_mode=4, request_mode=0, boxtype=pc).
I pulled tvheadend from git, ./configure and make. (no changes in the ./configure).
I disabled CWC and added a capmt (the Oscam button was there ;-).
When I play an encrypted channel I see in Oscam
2013/01/05 21:12:53 E02220 c dvbapi: Found 9 ECMpids and 1 STREAMpids in PMT
2013/01/05 21:12:53 E02220 c dvbapi: New program number: 0FAC (0500:0FAC unknown) [pmt_list_management 3]
2013/01/05 21:12:54 E02220 c dvbapi: Start descrambling PID #8 (CAID: 0100) 1
2013/01/05 21:12:54 E02220 c loopback (0100&00006A/5C00/0FAC/64:2EF9): found (304 ms) by omnikey-extra (P/2/2/2)
2013/01/05 21:12:54 E02220 c loopback (0100&00006A/5C00/0FAC/64:62B7): found (308 ms) by omnikey-extra (P/2/2/2)
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 0] CAID: 0500 ECM_PID: 0590 PROVID: 042500
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 1] CAID: 0500 ECM_PID: 057C PROVID: 042500
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 2] CAID: 0100 ECM_PID: 0644 PROVID: 00006A
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 3] CAID: 1819 ECM_PID: 0734 PROVID: 000000
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 4] CAID: 0100 ECM_PID: 0734 PROVID: 00006D
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 5] CAID: 1818 ECM_PID: 0720 PROVID: 000000
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 6] CAID: 0100 ECM_PID: 0720 PROVID: 00006C
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 7] CAID: 1817 ECM_PID: 070C PROVID: 000000
2013/01/05 21:13:04 E02220 c dvbapi: [ADD PID 8] CAID: 0100 ECM_PID: 070C PROVID: 00006A
2013/01/05 21:13:04 E02220 c dvbapi: Found 9 ECMpids and 1 STREAMpids in PMT
2013/01/05 21:13:04 E02220 c dvbapi: New program number: 0FAC (0500:0FAC unknown) [pmt_list_management 3]
2013/01/05 21:13:04 E02220 c dvbapi: Start descrambling PID #8 (CAID: 0100) 1
2013/01/05 21:13:04 E02220 c loopback (0100&00006A/5C00/0FAC/64:BB26): found (304 ms) by omnikey-extra (P/2/2/2)
2013/01/05 21:13:14 E02220 c loopback (0100&00006A/5C00/0FAC/64:89DB): found (304 ms) by omnikey-extra (P/2/2/2)
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 0] CAID: 0500 ECM_PID: 0590 PROVID: 042500
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 1] CAID: 0500 ECM_PID: 057C PROVID: 042500
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 2] CAID: 0100 ECM_PID: 0644 PROVID: 00006A
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 3] CAID: 1819 ECM_PID: 0734 PROVID: 000000
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 4] CAID: 0100 ECM_PID: 0734 PROVID: 00006D
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 5] CAID: 1818 ECM_PID: 0720 PROVID: 000000
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 6] CAID: 0100 ECM_PID: 0720 PROVID: 00006C
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 7] CAID: 1817 ECM_PID: 070C PROVID: 000000
2013/01/05 21:13:17 E02220 c dvbapi: [ADD PID 8] CAID: 0100 ECM_PID: 070C PROVID: 00006A
2013/01/05 21:13:17 E02220 c dvbapi: Found 9 ECMpids and 1 STREAMpids in PMT
2013/01/05 21:13:17 E02220 c dvbapi: New program number: 0FAC (0500:0FAC unknown) [pmt_list_management 3]
2013/01/05 21:13:17 E02220 c dvbapi: Start descrambling PID #8 (CAID: 0100) 1
2013/01/05 21:13:24 E02220 c loopback (0100&00006A/5C00/0FAC/64:CE94): found (303 ms) by omnikey-extra (P/2/2/2)
I have an oscam.dvbapi that ignores all but 0100:0000006A::
Oh, with CWC the same works like a charm. How did you get capmt going?