I usually trap my stuff with what ends up being a really dark color over a really light one... since i usually work with 2 color designs- its easy and i usually dont have to worry about the bottom ink showing through.
since i dont print though...im having trouble sepping something now. if my base color was a Process Cyan... and i covered it with PMS 675 (with the CYAN having about a 2-3pt stroke under it) how much would the Cyan show through, if at all? think it would be really noticeable?
Pantone 675 looks to require a good amount of transparent base, so I think the trap lines will be noticeable.
Your best bet is to make some color swatch tests. I'd print two cyan swatches, and two 675 swatches. On one I'd print the 675 swatch off to the side and slightly overlapping the Cyan (to more or less represent what the trap will look like).
On the other swatch I'd print the 675 directly over the other Cyan swatch. This will give you a good representation of direct overprinting. If the resulting color doesn't shift too much from the original 675, overprinting may be an option.
If the overprint doesn't work and you have to go with a standard trapping method (and the trap line will be obvious), here's one trick I've done in the past. Let's say you decide to use a 3pt trap... take the last 1pt-1.5pt of the 3pt trap then feather and delete the edges. Apply the highest LPI halftone you can hold (and print) to that feathered edge. As long as you're good at maintaining decent registration, the trap line will be way less noticeable as before.
you get the colors you have as PMS colors and place one on top of the other and then select the Transparent tool and make the color 50% (example)
It's not the best way but at least you wil have kind of an idea