Not knowing what version of Photoshop and Illustrator you are using, I can offer the solution that has been used for years... (and what you figured)
Instead of attempting a specific spot color, just use one of the basic plates (C, M or Y). Then tell the printer to subsitute this plate with your particular PMS ink.
The latest versions of your software offer other options. I think Photoshop 6 and above offers a spot color channel - - but you would still need to have your printer replace PMS xxx with PMS 877 on the press.
Does Pantone offer a color palette file for this metallic ink?... If so, just add to Photoshop.