I would however point out that all BTC LN hubs may not be able/willing to exchange BTC for LTC, so you may need to have your transaction go through a higher than normal number of 'hops' in order to get to a LN hub that is willing to do this.
I would also point out that a BTC LN hub will likely want a commission to exchange BTC for LTC (similar to how shapeshift.io takes a spread/commission), so going between blockchains might not be as frictionless as you imply.
More info here:
https://bitcoinmagazine.com/articles/atomic-swaps-how-the-lightning-network-extends-to-altcoins-1484157052/
Also Coblee's post about it:
https://segwit.org/my-vision-for-segwit-and-lightning-networks-on-litecoin-and-bitcoin-cf95a7ab656b
The way I understand it, it simply needs to be a hub that opens channels with Bitcoin and Litecoin users. Hubs will charge transaction fees, and they may decide to charge extra transaction fees for LTC users, but it is a free market, it's relatively easier to setup a LN hub than an altcoin exchange and there may be much less regulation, so there will be a lot of competition so it will likely be much cheaper than existing exchanges. LN will also take the best route. From an end users perspective, all they will have to do is type in a litecoin address and LN handles the rest. This is why BTC segwit addresses start with bc1, ltc will have a different prefix.
You are right that it is a free market, however I would anticipate that the market would result in LTC LN hubs that are willing to exchange ltc for btc will have both btc and ltc on an exchange and will trade ltc for btc whenever someone wants to exchange btc for ltc and vice versa (and of course will charge a premium above the trading fees on the exchange.