DNA/RNA random mutation method?

[Maximilian Haeussler]

>Problem with all of the above is that mutations will not be perfectly
>random. Some recent polymerases, OTOH, were tweaked to make
>nearly random mistakes.

RNA viruses achieve a highly random mutations ... I wonder if there a
mutagenetic technique involving transcription to RNA, mutation and
reverse-transcription afterwards...?
Erm, before cliking send, I typed it into google and found this:
http://www.biomedcentral.com/1472-6750/7/18 an RNA-based mutation strategy
based on Qb replicase.

I still wonder that, given that mutagens should be more effective on on RNA
than on DNA, why people don't rather mutate RNA...?

max