You can drill in the white plastic part, just don't get close to the black area near the pins. The PCB is embedded in the black area.
For practical reasons, a keyring is more functional if you drill a corner.
Unfortunately, for me, a HuCard is just too damn big for a keyring (see exhibit A). I have a hunk of plastic I took from Red Roof Inn 20 years ago. For me, it is the perfect keyring (sentimental + not too big + not too small + two decades of grime).
Of course, if you don't mind the large size of a HuCard, it would make an awesome keychain accessory. It is the perfect thickness (comparable to the thickness of Red Roof Inn's gold-standard, evidenced above).
However, if you drill a hole at the end of a HuCard, chances are that it will hang upside down for most of its existence. Therefore, I suggest that you defile the HuCard and drill into the black PCB area--if only to better appreciate the artwork. Compare
Saint Dragon (left) to
Ordyne (right).
Why stop at the HuCard, though? Use the entire jewel case as a keychain:
You can store a PCE manual, HuCard, notes, road maps, flan and/or sundry items in the jewel case itself:
The jewel case is an excellent case to hold your keys.