There has been discussion on this in the past but I do not recall if there was a definitive answer to the question or not.
Looking at both the manuals for the Valk and 98-00 it would appear that it may be possible to fit the reverse from the GL500A/SE to the Valk. The Japanese models apparently came with reverse and so it is somehow possible but without access to the parts fiches for the Japanese models it is difficult to say with any confidence without actually stripping down the transmission. It may be worthwhile comparing pre and post 98 Goldwings for parts interchangeability because if the older bikes can donate parts there will be more availability.
Here is a link to the reverse gear fiche (
http://www.hdlparts.com/fiche_section_detail.asp ). If you compare this to the Valk manual from the Norway site you will see that it looks like the same support is used for both transmissions (see page 10-1). There appears to be bosses for the reverse gear shaft to pass through and it looks like there is a boss for the shift arm installation. Someone that has had the transmission apart may be able to tell you if these bosses are machined to finished sizes or not.
This looks like it has the parts that you would need to do the job, you would have to do something for the selector to engage reverse and perhaps some safety interlocks (
http://cgi.ebay.com/ebaymotors/Honda-Goldwing-GL1500-ENGINE-MOTOR-BLOCK-TRANSMISSION_W0QQcmdZViewItemQQhashZitem1e5873178fQQitemZ130332956559QQptZMotorcyclesQ5fPartsQ5fAccessories ).