Try this link it may help you.
http://www.northernlightsembroidery.com/hats.htm

You could probably do it by opening the back strap and laying the side you want to embroider on onto a sticky stabilizer...I use a cap hoop for the front embroidery but have only ever put the name across the back....don't see why you could do the side easily with the sticky stabilizer...

I have put a picture of a baseball cap hooped so you can embroider across the back, not sure if this helps...go to projects ...

depending of the type of machine you have you might be able to use a special hoop that is intended to be used with caps.

I haven't tried doing caps yet. But I understand you can do it using hoopless method -- hoop your stabilizer -- open up the cap or item you want to embroider and position it smoothly over the stabilizer and hand baste it to the hooped stabilizer -- attach your hoop then proceed to embroider.

I use this method a lot since I do not use spray adhesive because of many allergies to chemicals and it works great for things I have done.

I'm also going to use this method for embroidery my market tote bags.

so far that's one thing I have not embroidered but you'll get gr8 answers from cutes who have *4U