The former ("proper 32-bit x86 package form") has been suggested
by zerg@ quite some time ago but the desired interface wasn't clear
at the moment IIRC; a quiet morning helped me realize that

  ICAClient-preinstall@IA32

is rather more readable than

  ICAClient-preinstall@i586 i586-ICAClient-preinstall@x86_64

so here's the (trivial) implementation; and I actually needed
the latter, @X86 ("any-x86") to mark x86-only packages so

  xorg-drv-intel@X86

is now equivalent to

  xorg-drv-intel@i586 xorg-drv-intel@x86_64