Pedro has written an excellent proof of principle Greasemonkey script that checks to see if any DOIs on a web page are in the Postgenomic database - a picture and a link back to the relevant page on Postgenomic is inserted after each match.
The code in question is up at Nodalpoint. At the moment it definitely works on Nature.com but with a little tweaking it could be adapted to other journals too (for example, looking for an optional space between the colon and the actual DOI allows the script to work at Sciencemag.org).

Pedro's script uses a cached list of DOIs from Postgenomic, rather than making repeated requests. Eventually the API will get off the ground and provide more than this (very basic) functionality but - for now - if you'd like to experiment with anything and need any support from the Postgenomic end just let me know.