How can I modify properties of a job (StartInterval, in this case)

without un/reloading it from launchd?

when I read "how do i modify x without reloading it" I see a design issue.
Startinverval " When an agent has the StartInterval key in its .plist file, so that it is run every N seconds"
from macgeekery .

which is a summary of the launchd manual

StartInterval <integer>
     This optional key causes the job to be started every N seconds.  If the system is asleep, the job will

     be started the next time the computer wakes up.  If multiple intervals transpire before the computer is
     woken, those events will be coalesced into one event upon wake from sleep.

Ideally, I want a daemon to be
able to modify its own StartInterval while running, so that it applies
the next time the daemon idles out, and without disrupting any of its
launchd-provided sockets.

Idleingout, I think referes to OnDemand, because you want to control when the job comes back after being stopped from lack of use? This is probibly not necessary.
again from the man page

KeepAlive <boolean or dictionary of stuff>
This optional key is used to control whether your job is to be kept continuously running or to let

demand and conditions control the invocation. The default is false and therefore only demand will start
the job. The value may be set to true to unconditionally keep the job alive. Alternatively, a dictio-nary dictionary

nary of conditions may be specified to selectively control whether launchd keeps a job alive or not. If
multiple keys are provided, launchd ORs them, thus providing maximum flexibility to the job to refine

the logic and stall if necessary. If launchd finds no reason to restart the job, it falls back on
demand based invocation. Jobs that exit quickly and frequently when configured to be kept alive will

be throttled to converve system resources.

SuccessfulExit <boolean>
If true, the job will be restarted as long as the program exits and with an exit status of zero.

If false, the job will be restarted in the inverse condition. This key implies that "RunAtLoad"
is set to true, since the job needs to run at least once before we can get an exit status.

NetworkState <boolean>
If true, the job will be kept alive as long as the network is up, where up is defined as at least
one non-loopback interface being up and having IPv4 or IPv6 addresses assigned to them. If

false, the job will be kept alive in the inverse condition.

PathState <dictionary of booleans>
Each key in this dictionary is a file-system path. If the value of the key is true, then the job

will be kept alive as long as the path exists. If false, the job will be kept alive in the
inverse condition. The intent of this feature is that two or more jobs may create semaphores in
the file-system namespace.

OtherJobEnabled <dictionary of booleans>
Each key in this dictionary is the label of another job. If the value of the key is true, then
this job is kept alive as long as that other job is enabled. Otherwise, if the value is false,

then this job is kept alive as long as the other job is disabled. This feature should not be
considered a substitute for the use of IPC.

Tell us why you want to do that and I could probibly give you a better design decision.