In reply to Moacs:
I've done it. We went with a guide called Brian Dawson who I believe has now stopped doing trips. Tom Bennett appears to have taken on his role.
The walk itself is a fun day out. On a sunny day the entire wet sand flat becomes highly reflective so it is like walking on a mirage. The distance isn't too far and the surface when we did it was firm, so not too tiring. Beautiful part of the world. Take wellington boots as it is v wet.
I'd recommend a guide. In good conditions the walk is reasonably banal from a risk perspective and I think even in foggy conditions an experienced mountain walker could probably navigate blind and avoid calamity, particularly if GPS-equipped. The guide took the stress out of the day, ensured the local conditions were good (hard to tell exactly how hazy it is on the day without being extremely local). Also good to hear some of the history and stories. It wouldn't be fun on a hazy day I don't think, the view is what makes it a fun day out.
I think you could make it to the museum and back if you moved at a reasonable pace. We were a large group that moved slowly and probably had time. But I'm not an expert on the local conditions and tide heights are very variable in this part of the world. I've been stuck on Osea island for hours before when the tide was supposed to have gone out but hadn't because of an onshore wind...