It's an interesting idea but I don't think it will work because, in terms of the MultiChain protocol, asset quantities have to be represented properly in transaction outputs, and with a P2SH the full script is embedded in a subsequent transaction input instead.
First I can understand a particular hardware wallet choking on OP_DROPs but I would not think that OP_RETURNs would be a problem, since is quite common in bitcoin. But I don't think that will help you here.
So I think you should use a signing library that does not explicitly check for standard transactions. You can also consider using multichaind-cold which is the cold node version of MultiChain that comes with the download.