Peeking inside the .ph file on my machines, it's mostly just more requires.

Yes, the "load a file only once" logic with %INC would apply to every require call. I don't have enough time to test right now, but assuming that those files don't load any other modules, an ugly hack might be { package Foo; local %INC; require "..."; }

By the way, I took a look at your commit, and I noticed this logic:

eval { require 'sys/ioctl.ph' };
$Tcl::pTk::_FE_unavailable = $@;
...
if ($Tcl::pTk::_FE_unavailable) {
...
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Be aware of Bug in eval in pre-5.14 and that $@ may not be a true value after an eval failure. The better pattern would be for example:

eval {
    require 'sys/ioctl.ph';
1 } or do {
    $Tcl::pTk::_FE_unavailable = $@||'unknown error';
};
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because in this case $Tcl::pTk::_FE_unavailable will always be a true value if an error occurs, even if $@ is not.