Page 67 of the regs states that a hunter that has a modern firearm tag may use a crossbow in a firearm resticted area.
Anyone or only a disabled hunter? 
Anyone in a firearm restricted unit during modern firearms.
http://wdfw.wa.gov/wlm/game/hunter/huntregs2009.pdfFirearm Restriction Areas
The firearm restriction areas listed on this page
have been established by the Washington Fish and
Wildlife Commission. Centerfire and rimfire rifles
are not legal for hunting in these areas.
In firearm restriction areas, hunters may hunt only
during the season allowed by their tag.
Archery tag holders may hunt during
archery seasons with archery equipment.
Muzzleloader tag holders may hunt during
muzzleloader seasons with muzzleloader
equipment except in the GMU 652
restriction area outlined for King County.
Modern firearm tag holders may hunt
during modern firearm seasons with bows
and arrows, crossbows, muzzleloaders,
or revolver-type handguns meeting
equipment restrictions, or legal shotguns
firing slugs or buckshot.