For what it is worth, your husband is only planning to grow the plants, not the fruit in the GH and if that is the case, you will not have to worry. LJA did the tomato from seed thing when we first built our GH. He also grew peppers that way. In the spring, we would put them out in the regular garden. The little plants did nothing that I could see or remember to damage the orchids. They do however take up space and the trays have to be kept warm when the seeds start to grow.

My understanding is that the ripening fruit is what causes the release of gas that could cause bud blast or damage if in close proximity to the orchid.

Cheers,
BD