I looked in the source you published to your proposals and saw
a switch (.if .ifelse ...) on ${OPSYS} there. Wouldn't it make
sense to look for a file, with a name derived from ${OPSYS} to
include in the case the opsys isn't knowen. It would make it easier
to add new things to it, no troubles with new version of "bsd.pkg.mk",
because its not changed.

Example:

.if (${OPSYS} == "NetBSD")
...
.else
.if (exsist(/etc/bsd.opsysconf.${OPSYS})
.include </etc/bsd.opsysconf.${OPSYS}>
.endif
.endif

I also thought that configure and extract should better use one
variable and swich on its contence, with hooks for unknowen values,
at my first look into bsd.pkg.mk (FreeBSD Version). For extract I
prefere your solution now, but for configure I'm not shure how many
new methodes will come up that it make sense to change a running
code.

Bernd Salbrechter

PS. I will not be online for the next week, but I will answer
replayes after my vaction.