ClickOnce is currently a very
undersupported technology in my
opinion. Currently only Internet
Explorer can natively execute it, and
FireFox if you use the FFClickOnce
add-on.
This used to be the case but isn't anymore: .NET Framework 3.5 SP1 does install its own add-on for this purpose (see .NET Framework Assistant for Firefox). Whether you consider this to be 'native' support or not is up to you, but of course it's still a regular add-on, though being officially released and supported by Microsoft helps a lot to improve Firefox user experience in regard to ClickOnce applications.
Obviously, in order to facilitate this new official Firefox add-on, the .NET Framework 3.5 SP1 must be bootstrapped by itself in case users don't happen to have it already, but this is another topic and doesn't differ much from installing any other baseline technology. It's worth nothing though that there has been improvements in this area too: By means of the .NET Framework Client Profile (see related FAQ too) many applications can reduce deployment size significantly (apparently ~28 MB instead of ~230 MB for the full framework).
Does anybody know of supported methods to open ClickOnce applications
from Opera/Safari/Chrome?
Other then some questionable workarounds I'm not aware of any robust solutions.
If not, where would one begin in
creating a usable, robust plug-in to
achieve this functionality.
Your first stop regarding ClickOnce development (available for .NET 2.0 and higher) should be ClickOnce Deployment for Windows Forms Applications (the name is misleading, ClickOnce is not limited to Windows Forms). You'll find a wealth of information there on how to use the ClickOnce API, both implicitly via the build in deployment tools and especially programmatically from your own applications. The latter seems to be rarely used so far, unfortunately, though it is actually quite powerful. Consequently you should be able to include this functionality in almost any kind of 3rd party application, including browsers of course, as long as those do provide any kind of plug-in architecture.
Where would one start in making
FireFox, Safari and Opera extensions?
I understand (and applaud) your approaching this issue from the single angle of end user ClickOnce experience, still I think given the broad scope of the topic it should best be asked in a separate question, if not in a separate question per browser even. That said, there are quite some related questions to be found on Stack Overflow already, see e.g. for Firefox (most obvious ;) questions/tagged/firefox+add-on and questions/tagged/firefox+extension.