RPC: Shielding of part of transparent balance not possible

[buck54321]

I'm trying to maintain a "trading account" that a user can trade with on a DEX, but also allowing users to send some funds to Orchard in a different account. The somewhat confusing docs say.

Change generated from one or more transparentaddresses flows to a new transparent address, while change generated from a legacy Sapling address returns to itself. When sending from a unified address,change is returned to the internal-only address for the associated unified account.

What exactly is the internal-only address for the associated unified account? If that means it will return change to the Orchard pool over transparent in the unified receiver given for the fromaddress argument, then it is now impossible to send part of a transparent output, because it is impossible to generate a unified address without a shielded receiver. So if I have say one large utxo of 10 zec, and I want to send half to the orchard pool and keep half for trading on the DEX, I can't do that, because I must provide a unified address and a unified addresses must contain shielded receivers, and when a unified address is used, apparently the change goes to the shielded receiver.