Except that a Launch Daemon (like a Startup Item before it) suffers from two important problems in this regard:
If System Configuration redoes the network configuration, it will trash your route. You can combat that by making launchd run your Launch Daemon periodically, but you might still have to wait for that length of time for your route to be restored.
Both Launch Daemons and Startup Items can run before some network interfaces are fully configured. In that case, you may not be able to make a route at that time, and might have to fall back to waiting for a periodic invocation of your Launch Daemon (or in the case of a Startup Item, perhaps for a cron job).
I just posted what will hopefully be a better (i.e. more robust) solution on my website. I built an installer package (download) for it; there's also some source code, but I can't post the link on Server Fault as I don't have a high enough reputation yet to put more than one hyperlink per post.
To use it, install the package (so far only tested on my laptop, but appears to work). It should install a Launch Daemon that starts the "staticrouted" server process (you can check this in top; if it isn't running, it probably means I got the installer slightly wrong, but you should be able to start it using launchctl as usual).
To create static routes, you use the "staticroute" program (which you'll find in /usr/local/sbin). e.g.
staticroute add 10.128.4.0/24 "Main Network Interface"
staticroute delete 10.192.0.0/16 "Other Interface"
You can also see the list of configured static routes with
and a list of interface names with
Configured static routes survive a restart, and will automatically be added and removed as network interfaces go up and down.
There are also man pages; these are installed in /usr/local/share/man/man8, so you might also want to add /usr/local/share/man to your MANPATH if it isn't already there.
I should emphasise, I just knocked this code up today to fix a problem that is being caused by my having to use a cron job to keep static routes in place. It appears to work for me, but YMMV. The code and the binaries are MIT licensed.