My understanding of the current FW class rules is you can do whatever you want to the top of the board, as long as you don't remove the serial number. Here's the rule:
C.5.2 MODIFICATIONS, MAINTENANCE and REPAIR
(a) Alteration of hull shape, major underside re-fairing or deliberate
removal of the finishing coat/s is not permitted unless in the case of
the result of local repairs to unintentional damage. The serial
number as required by D.1.1 shall remain legible.
'Altering hull shape' is pretty open to interpretation. I can't see it as being a problem as you aren't touching the bottom.
I tested double chicken straps on the Gaastra Vapor board quite a bit this year and it's a fairly similar width to the HWR in the tail (compared to the larger boards like the JP and PD, which use double straps). I loved it! I've always thought having the strap further forward than the fin means you don't load the fin correctly in high winds and promotes spin-out (exactly what you DON'T want in strong winds) - however, from a manufacturing/marketing point of view it's more convenient to have the strap using further forward so it requires one less plug and less reinforcement which keeps the weight of the board down, so I can see why all the brands don't make this double-strap as standard.
I built a plate myself using the idea from that forum post ardenalan mentioned and used it a lot at the GP in Brasil the other week. You can see it here: http://www.twitpic.com/372ptm
You can generate much more leverage on the fin when have the straps further out than the centre of the board which means not only can you survive in strong winds, you can actually go REALLY fast downwind compared to a conventionally placed chicken-strap. I discussed this with a lot of guys in Brasil and a lot of people shared this view, although most guys aren't handy enough with carbon to build a plate.
The other benefit is you can start using the chicken-strap much earlier (in lighter winds) as you can still rail the board easily because you have your feet further out than the original strap was. So it's a win-win. The plate I used hardly weighs anything, however putting plugs in would be better as the straps on the plate sit up 1.5cm from the deck so even with the smallest straps I could find on the market, they are still a bit loose because of the height of the plate.
Remi would be right about the reinforcement area though. I would think if you added the plugs yourself I would be adding some extra layers to that whole deck area to keep them in.