I'm would think that manufacturers publish the "RF current ratings" of their ceramic disk and silver mica dipped capacitors. However I can't seem to find this info. I would expect that the maximum recommended current is derated as frequency increases, and as the DC bias is increased on these capacitors. My interest is capacitors for HF QRP to 100 watt RF output HF amateur transmitters and 50 ohm half-wave low-pass filters. Thanks -

You'll find information on "current rating" considerations for capacitors in the following Q & A info sheet:
http://www.arrl.org/tis/info/pdf/9701085.pdf