It appears to DGP that many people's concerns and troubles with writing index scripts (those bits of code most often found in files named pkgIndex.tcl) come from approaching the problem in the backwards direction. Learning to think in the opposite direction is a better idea than inventing a lot of patchwork to support the wrong direction, like...
(Not to disparage that effort at all. Dealing with platform names in a standard way is an important bit of infrastructure, and it's good to see efforts in that direction. The problem is that index scripts are the wrong place to confront that problem.)
Perspectives: providing a package vs. requiring a package
More to come...