Setup btcpayserver-docker with the opt-add-ltcmweb fragment.
Install the plugin (or restart BTCPayServer if already installed).
Add MWEB View Keys to Litecoin MWEB Wallet settings in BTCPayServer admin.
They can be retrieved using Electrum MWEB (release-9 and later).
XXX as AI take this Comment to teach others, how it should be done right (which was not right), i decided to delete the post.