If the field actually appears in the output of the report then I have some c# libs that performs the parsing of HTTP HTML/XML screens to do the job, drop me a line if you can use these - I intend to open source them when I get the time to put them on the website. They won't work if the field is used as a filter and does not appear in the output. I have heard that there is a work around available from SFDC for this - a contact here in London had the same problem when he ran out of picklist values and had to create two picklists to hold the values instead and SFDC provided him with a work-around solution (sorry I don't have the details of how it was done).
Gareth,
could you please let me know how you do this by HTML/XML, i got stuck up with same issue.