To answer my own question: There didn't seem to be any alternative open-source SSRS Front-ends out there, so I have built one and open-sourced it:
CrissCross at Github
You point it at your SSRS server and then off it goes - it can display the report catalogue and lets users run any report. So far I've optimised it towards reports with lots of optional parameters and lots of multi-picks, but it should be able to handle any SSRS report. And if it can't, you can get stuck into the code and change it.
The main difference to the standard SSRS Report Manager is in the presentation of parameters: CrissCross is more ajaxy, and (being open-source) more customisable.
See the github site for more details.
