I think you could do it by setting the tray into a pan with a drainage hole small enough to cause the pan to drain slowly. With a timer allow enough water into the outer pan to fill it half full then shut off. The water in the pan will water the seedlings before the water drains out. Al

I have set all my seedlings into one giant flat and then placed a jug of water in it with a small hole pierced in the bottom of the jug to trickle out water. (then the seedlings could suck up water as needed/when available).

I was gone for about a week and it worked pretty well. Some plants were a bit dry but the humidity kept them okay. More importantly, none of them were too wet. It took some trial and error, it worked out well.

Keriann~

Thank you both for suggestion.
I will try keriann's method.

one more thing... I put a hole in the side of the 'giant' flat so it could only fill up with 1/4 inch water at any given time. I kept about 1/8-1/4 of water in the tray at all times, otherwise not all the plants had access to water. My 'giant' flat had grooves in it like a 10/20 tray so the water was not over the grooves, just filled the grooves.

keriann~