Solved the problem without resorting to udev using the info on the MythTV wiki.

And added my solution to that wiki page. Copied here:

Another example (added November 2009, using MythTV 0.22 with a 2.6.31 kernel): a backend with two Hauppauge Nova-S-Plus DVB-S cards and a Hauppauge Nova-T 500 Dual DVB-T card. Adding these module options to /etc/modprobe.d/dvb.conf:

options cx88-dvb adapter_nr=0,1
options dvb-usb-dib0700 force_lna_activation=1 adapter_nr=2,3

makes sure that the DVB-S cards are always adapters 0 and 1, and the DVB-T card’s dual tuners show up as adapters 2 and 3. This is confirmed in the system log, where the adapters are numbered correctly despite being detected in a different order:

$ dmesg | grep "registering adapter"
[    7.163022] DVB: registering adapter 2 frontend 0 (DiBcom 3000MC/P)...
[    9.840541] DVB: registering adapter 0 frontend 0 (Conexant CX24123/CX24109)...
[    9.841996] DVB: registering adapter 1 frontend 0 (Conexant CX24123/CX24109)...
[   10.522401] DVB: registering adapter 3 frontend 0 (DiBcom 3000MC/P)...