Hi Pan. Yes, i do this with very young seedlings. You can wait till they get a bit bigger.
Since we are using epi's for grafting, any time is good...the seedling are very activelyy growing and will heal very fast. Yeah, it's true, big avocados are hard to graft because of the lack of healing cells...when they are young, they heal very fast. Please keep an eye out, to not girdled the graft, I almost lost a plant, because of this. Grafts, when healing they expand to complete the union
The right time to graft is when the seedling, has the same thickness of the scion. I usually graft during spring to summer...if you have a greenhouse, to start the seeds...you can graft year round