As an alternative to skinning parts I'm now looking at making covers using resin infusion, however I need to add thickness to my original parts to create the molds.

I know wax sheet it generally the done thing, however I've read about issues with conforming to some shapes and corners, and some of the parts I'll be making have quite sharp corners (which would be fine in a mold but I anticipate issues getting the sheet wax to conform to the shape whilst also maintaining an even depth/thickness).

Has anyone ever used pattern coat to add thickness to a pattern/original part for this purpose?

I'm guessing that you'd need to be expanding the thickness by, at least, 1mm so using a brushed-on coating like pattern coat primer would likely need quite a lot of coats. Getting things to a consistent depth afterwards sounds like it would be very difficult indeed, with a possibility that the coating might easily break away when de-moulding also

There is also the option of covering the original part with fibreglass matting, which should conform well to tight returns but, again, it sounds very difficult to ensure a consistent depth overall when is comes to coating and finishing

The 1mm sheet wax is very compliant when warmed-up a little and can be eased around most tight angles without over stretching or risking it breaking. However, some 'dulling' of the angle where the depth might creep a little over 1mm might no be too bad a thing, depending upon the process you intend to use when making your covers?

If you're intending to use pre-preg for the final part, getting the fabric into tight corners whilst maintaining the thickness of of the laminate at 1mm or less all over will probably be easier than if using infusion and definitely more likely than if using a wet-laid process, where some slight bridging is likely. Going slightly above 1mm depth difference around tight corners might actually be helpful and give you some slight leeway when it comes to making the parts. Something to bear in mind if the part is particularly complex, as getting a good fit might depend on the ease of consolidating the fabric.