Originally Posted by
madpogue
Unless there's something I'm unaware of, "tubeless" does not refer just to the tire. The rim has to be designed to allow the airtight seal of a tubeless tire. Old-school 27" rims were never imagined to be used with tubeless tires. So it's a matter of design, not just size.
Nitpick, but the rims don't need to be designed for tubeless as long as they are of reasonably high quality. There's a technique often called "ghetto tubeless" where one applies tubeless rim tape to a standard non-tubeless wheel, and this will often succeed if both the wheels and tires are of sufficient quality.
In fact, Stan's even makes a kit precisely for ghetto-izing non-tubeless rims.
https://mikesbikes.com/products/no-tube-rim-strip
None of that applies to a 27" setup, however, because none of the tires are tubeless-capable. You could maybe get one to seat on the rim, but it wouldn't be safe or smart to ride.