I rebuilt a set of 28sp forks a while ago for another forum member, they had seized the adjusters like yours and had fun and games stripping them. The alluminium fork top had corroded in the bore where the Brass adjuster goes through and caused it to seize.

I had to completely strip the adjuster assemblies and clean all the corrosion out, the blue knobs are held onto the brass adjusters with a tiny allen screw which rounded off so had to drill them out and retap them for a bigger screw.

Honda dont sell the parts seperately, you have to buy the complete damping adjustment assembly, had a quote from Lyngs Honda on these and I think they were about £150 plus vat each if they are still available to buy.

I think your options are to either get a 2nd hand one, Yahoo auctions Japan is the best place but 28SP forks fetch good money (£300 to £400) or take the broken parts to a local machine shop and get some made.