Its snowing moderately here (15miles from Rhayader), so there
s plenty about on the hills now.
Ref Elan Valley if you want I could take you around a few of the trails there, (email me on that), as I do some MTB guiding over in that area anyway. Although most of routes can be readily navigated at points routs choice of sheep tracks isn`t so obvious! However given the longer term forecast, and possibility of further snow and ice, Elan Valley higher tops maight be rather difficult indeed.
Nant yr Arian will be about 45 min from you, but that too may be snowed up!
If you`re staying at / near Newbridge you also have the option of rides on Radnor Forest hills or the Gwaunceste / Llanbedr / Begwns hills areas, which have a good range of mostly grassy natural trails, plenty to go at but as above weather may well have upper hand again this winter (:
Re you last point, last winter here I couldn`t access most of the areas from late Dec to end Feb due to either deep snow on hills or highgly dodgy away from main road conditions (= untreated packed snow and ice)