Hi Stephen,
Mainly text and line work. A couple of logos. We only have Acrobat Pro 9 on Macs and Adobe CS5
I have not done this for a while and I may be a little off in my recollection. If you only have Adobe apps, then I have a hack for you. Depending on content, this may or may not work.
Convert a copy of the PDF to grayscale mode. This would be printed in the required spot. You could also darken blend in colour channels as suggested below into the grayscale conversion, in order to "beef it up".
or
Create CMYK .ps seps from the 4C PDF.
Turn the seps into PDF files.
Place the 4 seps into an InDesign document.
Black first. Then layer the CMY over the top of the K.
The blending mode of the CMY should be set to darken.
You may need to adjust opacity of the layers.
You may also need to swap around channels as the base and darker blend.
The final result is black only, single separation, which would be printed in the required spot colour.
Another way I would tackle this with only Adobe software is to load in the fonts used in the job or outline the fonts in a copy of the PDF. Then I would open it up in Illustrator and Photoshop as needed to change the colour, ensuring that nothing goes wacky in the conversion (yes, I know that this method is against best practice suggestions).
Best,
Stephen Marsh