I'm using mugi-urushi now, so I can answer that question @joXn asked about celiac and wheat-based urushi. My teacher thinks that it would be ok initially, because there are multiple non-wheat-containing layers of urushi on top of the mugi-urushi. But he thought over a long time, gluten could theoretically leach out as the outer layers wear down.
Historicay mugi-urushi was less likely to be made with wheat, so I think you could circumvent this entirely by using rice flour at the cost of moderately weaker strength.
cleaned up, then filled with sabi urushi. it's dark so it doesn't show up well, but the big w shaped gap is still mostly open in the centre so it can still cure