Off-the-shelf non-smart Motion Light/Sensor made smarter Phase II - Bouncing ideas here

For some time I have had one of those solar powered multi-light module Motion Lights hacked where the internal cavity of the largest light module was gutted and used to house a small lower power DC relay and a CR123 battery operated wireless alarm system door/window sensor with screw terminals for external switching. The relay was only on when the light was triggered on by it's motion sensor and this then closed the contacts in the said alarm sensor.

Worked great, except...well....every year when the 3V battery on the alarm sensor kicked the bucket and this light unit is up high and the whole thing has to be brought down and disassembled to get to that battery. So I considered this a proof-of-concept that I now have time to refine. The great thing is that I was getting the benefit of a purpose-built outdoor weatherproof and good ranging motion sensor AND a solar panel package.

Phase II:
What I'd now like to do is to replace the alarm specific sensor with a Zigbee door/window sensor that I can power off the light module itself (a 3-3.6vDC source) and ONLY when the light becomes active. So....

I could keep the above mentioned low power relay in the circuit and the series of events would be: motion sensor triggers lights, relay powers up closed, zigbee sensor powers up at same time as relay and then sees a closed trigger. Granted the zigbee sensor would take a moment to re-join the mesh but I'm thinking that within 15 seconds an HE rule would be triggered off that sensor.

I'm wondering if the relay could be totally done away with saving space and power....
that just merely powering that zigbee sensor up (as if it were one of the motion sensor's lights)
and either

  1. having it come online for HE to see (as a triggering event, haven't looked into this)
  2. having it come online in a closed (triggering) state
    might be all that's required.

Yes, this Zigbee sensor would be coming and going from the mesh, and yes this could present unexpected problems with it's path back to the HE. And certainly rule accommodations for repeated triggering within a short time span would be necessary to avert nuisance (as most of those solar light modules only stay lit for a few minutes at most).

But I'm asking for thoughts on what gotchas people see here. As I said this worked well with an alarm system that was told to "pay attention" to that particular Motion Light housed sensor during certain hours...and WAS ALWAYS powered by it's own battery and in communication with the base station. So there's a big difference.