I don't know your job process but are you sure your print supplier asked for spot trim marks? I deal with a dozen large press companies and none of them require anything other than the default cmyk trim marks that the PDF export puts on by default; this is because they are in the bleed area and not going to be in the final product.
Having said this you could plot lines in the bleed area and apply a spot colour to them (I do this for die-lines which you must turn off as they are in the art). However, won't you have to pay for a plate just for trim marks in this case?
Brett, I believe that sniper *is* a print service provider…
Sniper, do you use PDF workflow software or do you use Enfocus PitStop Server (fully automated) or PitStop Pro (semi-auto)? If you do, you should be able to do this without too many problems. I would hope that Callas pdfToolbox would also be able to handle this too as Adobe license their preflight technology.
This can be achieved manually/semi-automatically with Acrobat Pro 9, however in X and XI Adobe broke the preflight fixup that handles “Registration” colour (which is not really a colour, but a command). I don’t know if they fixed this up in Acrobat Pro DC.
It is possible to monkey with the default InDesign printer marks, however I can’t find any info on using a spot colour:
Also take a look at this topic thread for custom scripts: