Yes Alan, the manufacturer didn't allow room for a tripod mount. I'd like to think that they didn't deem one necessary but they may have never thought of one. I find it cumbersome to do macro work without a tripod mount.
When I used the Canon 100 macro, I placed an order with my local dealer. It took Canon Germany almost a year to supply the item! It seems that it was out of stock in Germany - this was some eight years ago.
There's a Germany fellow near Berlin who build handy items in small volume, also a tripod mount for a number of Nikon an´d Canon cameras but only for those, which are designed to have one mounted. He doesn't have a web site, couldn't handle an increase in volume. But his tripod mount is a delight to use with the 100 macro.
I now use Canon's wonderful 180mm macro.
A shame, that so few manufacturers find tripod mounts on macro lenses necessary.