[lifthrasiir]

> Does JPEG XL allow encoders to switch between the DCT and modular modes on a per-macroblock basis, or is it just on a per-channel basis?

Tricky but it can be indeed done on a per-macroblock basis. The encoding itself is fixed per frame, but JPEG XL mandates zero-duration frames to be merged with the prior frame, so multiple frames with different encodings can be used for that. In fact I believe patches already work like this.