You're right @MMMmmmm, I did get a little side-tracked earlier, you asked a very specific question and got all excited in using my new toy in Tasker.
You are also right to point out that passing the info through to HE at the time the alarm is changed and letting HE handle it from there does take the phone out of the mix then and removes the need for it to be powered and on the network, i.e. that is a good thing. So my call to flick a switch from Tasker is flawed in that sense.
My only concern with sending minutes till the alarm is any sort of delays, etc, that may introduce some innaccuracies, but the more I think about your use case, adjusting lights over a period does not need to be an exact science.
So the interesting development, though only partially complete... I have setup a new driver that may prove useful. It can store basic data types through calls to commands on the device. This could be called via Maker API like I described previously, with the addition of a parameter value in the command URL, i.e. the number of minutes until an alarm is due, stored as an integer value. All I have done at this stage is develop the driver and tested setting up a device, I haven't made use of it in Maker API, etc. Happy to do that as well, but if you are interested the code is linked from the thread below, you are welcome to take a look and see if it will suit your needs.
Simon
