as an addendum to the previous poster's info, the way the glass plates can be made food safe is, you paint and decorate the underside, leaving the eating surface plain glass, and then put the protective sealant over the artwork, on the underside of the plate only. That way the only thing that touches your food is glass.
For the wood, be aware that all wood finishes except beeswax (as in, melt or soften pure beeswax and rub into the wood, then remove as much excess as you feel you need to. It isn't knife-proof, but will keep liquids from seeping into your food) are toxic because of the solvents they contain. Even "pure" tung oil isn't pure: on the label you will see that it contains something like aromatic hydrocarbons. Very very toxic. If you could procure actual tung nuts, they are very oily and maybe then you could use that oil. But anything at the hardware store for finishing wood is hazardous to ingest or have contact your food. If the finish smells like a gas station, you know right off not to inhale it again or ingest it, etc.
If you want to use wood for historical reasons rather than glass as mentioned above, you could treat your plates (or trenchers) like chopping blocks, and oil them with vegetable oil and wipe dry, and clean with soap and hot water and then re-oil with each use. but that won't help with a painted design. If you wanted to get fancy, and more practical and possibly more historical than a painted wooden plate, you could carve or burn your design into the plates on the rim only, if they are the kind that have nice flat rims, and use veg. oil or beeswax as a finish, keeping the food off the rims if you don't want to scrub anything out of the carvings.If color is really important one could consider inlay with natural materials such as shell or bone, but that's way beyond the skills of most of us ordinary folks.
Just ideas, hope you find a good solution. I thought of the do-it-yourself clear polyurethane, but again, that is a plastic that you would get bits of in your food if you used utensils at all, and besides getting scratched and gouged and ugly, it's unhealthy. Probably an unpopular thought, but did people who displayed their coat of arms generally eat from wooden plates anyway? On the other hand, did people put coats-of-arms on plates that were actually for eating and not wall display anyway? Well anyway, I guess re-enactment has to be equal parts fantasy and factualness or it's no fun, so forgive me for being pedantic. Hope you find a workable solution and have a great time.