Thanks for bringing this topic to the forefront.
We have Progressive Insurance and we recently hired Grant Wilson (FreedomVanGo of Jacksonville FL and ABYC professional installer) install Epoch LifePO4 batteries and Victron BMS on our boat.
We talked directly to an underwriter at Progressive (after 20 minutes of "...that's a good question, let me get you to..."). We confirmed that physical losses and liabilities CAUSED by our installation would CURRENTLY be covered losses to the extent of our policy limits. The underwriter mentioned that at Progressive there was no trending risk related to lithium battery sourced fires and no current initiative to require additional riders or additional requirements on Progressive policies.
The underwriter did declare that any claim review will require documentation when major systems are modified from OEM. She recommended to continue to use professional ABYC installers and keep all documentation to keep our claims process smooth.
Peace of mind for now but warrants a close watch when we modify our systems...