Thus I went in search of open source tools that would make this happen. I first checked out OpenOffice since I used OO Impress to create the poster. However in addition to not having options to export to a CMYK pdf, the new version has screwed up all the poster font layout.
I decided thay instead of screwing around with that, I’d attempt to convert my existing pdf directly. Enter PStill, which once set up correctly made my A1 poster CMYK… booyah!
(width and height parameters may need to be changed, the important option is -m XimgAsCMYK which says to convert to CMYK, the other options are from the default example that comes with pstill)
UPDATE: Turns out that printeries convert to CMYK before printing anyhow. The problem with my poster is that the particular range of colours it uses can’t all be adequately presented with process dye printing (or something like that). Next time I’m using CMYK colour from the start.