libexec/rc/rc.d/ldconfig
6	I suppose FILESYSTEMS makes /usr/bin available, am I right?
25	I prefer the `$()` syntax, and I see no reason not to use it there. machine_arch below is assigned this way.
45	Besides checking the arch, we need to check that /libexec/ld-elf32.so.1 is present and is executable. System might have been installed without compat32 userspace tools.

libexec/rc/rc.d/ldconfig
6	`FILESYSTEMS` is a "dummy" dependency that requires `root`, `mountcritlocal`, `cleanvar`, and `tmp`. The `rc.d` scripts that explicitly invoke tools in `/usr/bin` (in `main` as of just now) are `bluetooth`, `iscsictl`, `jail`, `opensm`, `sendmail`, and `sshd`. Four of the six also `REQUIRE: FILESYSTEMS`, and no other `REQUIRE` target is at least as common within that subset. `mountcritlocal` mounts everything except (nominally) NFS and SMB. Without further analysis/testing, evidence implies that `/usr/bin` is available to the `ldconfig` script.
25	Four existing assignments use backticks (`_files` and `ldconfig_paths` in the initial loop, and `_files_` and `ldconfig32_paths` in the "64" case). One existing assignment uses `$()`. I will be glad to use which ever style you think is best, and would appreciate any insight about what underlies each style and your preference.

libexec/rc/rc.d/ldconfig
25	`$()` is better than backticks because it allows to clearly parse the nesting, and clearly see the missed delimiter. Single \` could mean either missed starting backtick, or missed closed backtick. Also backticks cannot nest. I believe this was the reason why ksh added `$()`.