Hi Ed,

< I have projected text onto a curved surface and extruded the text, but was unable to cap the ends because they are not planar.>

I'm often needing to do similar things with text for some of my jewellery designs.
Firstly you'll need to use your Projected text curves to Trim that curved surface you've made, then select those surfaces & extrude.
Just make sure you have the Cap ends checked.

HTH, Colin