I understand there's no smart contract layer but are HTLCs something that can be built into the base protocol? My first guess would be "no" because of differences between UTXO and Nano's account model where there's no concept of "spending" a coin acting as a payment invoice.
Such a function would allow for submarine swaps, which is a key feature of Bitcoin's Lightning Network. This allows a trustless swap with another chain that can support clawbacks (e.g. trustless swap of BTC with LTC, or BTC with XCH).
Here's an example of how a submarine swap would work cross-chain (between BTC LN and XCH): GitHub - richardkiss/chiaswap: Trustless atomic swaps between Chia (XCH) and bitcoin lightning.
Imagine being able to do a trustless swap of XNO for LTC!