Small recorders like the h1 or DR-05 can be hidden on an actor and synced up to video with pluraleyes in theory. But it's hard to turn them on and off for each take so you end up with a big file with all the takes that is hard to sync to multiple video files.

Wireless into a seperate recorder is much more convenient for the editor because you typically get one file per take, matching the camera file. And perhaps more importantly the audio recordist can monitor levels and make sure the recording is going well during the shooting. The audio quality can be poor due to interference or just the compression involved in companding.

I was thinking about combining both approaches to get the best of both worlds: put a small recorder on the actor with a lav mic plugged in, and route the headphone output to a wireless transmitter so the sound person can monitor levels and sound of the recording, and record a backup.

I'm curious if anyone has tried this and has any war stories. One issue would be getting a cable that feeds the 1/8" headphone out into the wireless transmitter. The senn g3 cable included with the kit with the 1/8" plug is designed for taking the output of the receiver, not feeding into the transmitter. There was a thread on this previously.

I have not done this, but a couple of things come to mind. As you stated you would need the proper cable to go between the output of the recorder and the input of the transmitter. Since routing the output of small recorders to the mic inputs of dSLR's is popular now, there are numerous cables available that could be modified or used in combination with the appropriate Sennheiser cable.
However, I would be cautious about both devices being in the same pocket. The recorder is likely to create a substantial RF "shadow" that could cause even more chance of dropouts than are already present by being in the pocket of a big bag of walking salt water (otherwise known as the talent).
Also, these small recorders may be susceptible to picking up RF interference from being in such close contact to the transmitter.
At the least, I'd put them on opposite sides of the actor, but that would require either routing the cable or using two very small independent lavs (which would give you full redundant audio but would require more careful rigging).
It would definitely need testing to find out any problems ahead of time.

I found it's very reliable to use a wireless lav on the talent and record it into my camera. If your camera lacks proper audio recording, then you are forced to use a dual system There you need a sound op who can also record. That I think is the weak link (human error) that maybe you are trying to fix.

A wireless transmitter such as the G3 is mic level impedance. Headphone outputs are line level. You will have to attenuate the signal in the bodypack. The G3 has this. That cable that comes in the kit should work fine but it only screws on at the body pack end. The other is susceptible to being yanked out of the headphone jack.

If one is to be considered back up or safety, then it might be better to keep the two independent of each other. If the recorder has an issue the wireless will not be affected.
Another issue is volume control. If the recorder is set for auto level control, or a limiter, this could be good or bad.

A wireless transmitter such as the G3 is mic level. Headphone outputs are line level. You will have to attenuate the signal in the bodypack. The G3 has this. That cable that comes in the kit should work fine but it only screws on at the body pack end..
Les is correct except the included Sennheiser 1/8" to 1/8" is wired for the receiver to feed a 1/8" (stereo) mic level input and will not work correctly with the SK-100 bodypack transmitter at line-level. However, an 1/8" input plug or cable can be configured to work @ line level (nominal -10dB) with the SK-100 G2/3 transmitter though. The (locking) 1/8" TRS input plug, must wired so the signal is sent to the 'Ring' connector; the 'Tip' must then be tied to the sleeve/ground. Again this is for -10dB unbalanced input...
That said.. I would use two mics and not interconnect the two devices... though theoretically it should work with a properly wired transmitter input cable.

I would suggest a look at the Zaxcom wireless systems. Their transmitters can be ordered with a built in recorder, no wires, plus the units can record TC and be controlled wirelessly. The transmitters incorparate a 2.4Ghz receiver for control.

David