Quoting Thor Andreassen <ta(a)toggle.be>be>:
I've failed to turn up any evidence that it is
possible, if it is, could
someone please point me to a plugin that does this?
Steve Harris has a "Decimator" in his swh-plugins that reduces sample
rate : "Decimates (reduces the effective sample rate), and reduces the
bit depth of the input signal".
There's also a "Smooth Decimator"
As for increasing sample rate Ardour (at least) automagically
upsamples imported sounds to match the sample rate of the project.
Alternatively you could use sndfile-resample or sox to do that.