Hi,
I've been printing direct from CO10 onto my shiny new Epson P800 and been very pleased with the results. Realise your post is from a while back now, but happy to help if you're still having issues.
Not quite sure from your description whether the aspect ratio of your image already matches the aspect ratio of your matt? Are you saying that your image is, for example 3x2 ratio, but you're wanting to print into, for example, a 4x3 ratio to suit the matt?
I don't think you can get Capture One to crop your image within the print dialog. You can setup your desired cell size (e.g 8"x10") and then if your image doesn't already fit that cell you can get CO to 'zoom' the image to fill the cell - but this stretches it in either height or width as required.
My approach is normally to create a virtual copy of the image ('clone variant' in CO terminology) and crop it to the aspect ratio I need. This way I get full control over the positioning of the crop. In the print dialog I can then set my cell size as needed knowing thjat the aspect ratios will match.
Does any of that make any sense?
Thanks,
Andrew.