It's no longer happening, and I'm not sure why. At this point, I think you can close the bug. I'm pretty sure it's not an apache issue.
For curiosities sake:
I had tracked it down to something in php5 startup on reload:
Breakpoint 1, 0x00007fefdb992180 in php_module_startup () from /usr/lib/apache2/modules/libphp5.so
(gdb) call (char*) getenv("APACHE_LOCK_DIR")
$1 = 0xffffffffdfa36a50 <Address 0xffffffffdfa36a50 out of bounds>
(gdb) bt
#0 0x00007fefdb992180 in php_module_startup () from
/usr/lib/apache2/modules/libphp5.so
#1 0x00007fefdbaa24d5 in ?? () from /usr/lib/apache2/modules/libphp5.so
#2 0x00007fefdbaa30c3 in ?? () from /usr/lib/apache2/modules/libphp5.so
#3 0x00007fefdf52ff71 in ap_run_post_config ()
#4 0x00007fefdf510dc7 in main ()
(gdb) c
Continuing.
Breakpoint 2, 0x00007fefdb9f7070 in zend_startup_modules () from /usr/lib/apache2/modules/libphp5.so
(gdb) call (char*) getenv("APACHE_LOCK_DIR")
$2 = 0x0
(gdb) bt
#0 0x00007fefdb9f7070 in zend_startup_modules () from /usr/lib/apache2/modules/libphp5.so
#1 0x00007fefdb992915 in php_module_startup () from /usr/lib/apache2/modules/libphp5.so
#2 0x00007fefdbaa24d5 in ?? () from /usr/lib/apache2/modules/libphp5.so
#3 0x00007fefdbaa30c3 in ?? () from /usr/lib/apache2/modules/libphp5.so
#4 0x00007fefdf52ff71 in ap_run_post_config ()
#5 0x00007fefdf510dc7 in main ()
(gdb)
q
So, I thought I would build a debug version of libapache2-mod-php5, so I could step through. I installed the build-deps (which upgraded several of my existing packages), and then I installed the current unstable version to verify that the release version was still causing problems. At this point, reloading worked; I downgraded the php packages back to what I had originally; and that also had no problems with reloading.
I hypothesize that one of the libraries I upgraded when I installed build-deps must have had a bug that was wiping out the environment, but I don't think I'm going to try to figure out which one. :(
Thanks for your time!