I'm the author of the Lost Cities mod and I would like to have an extension so that I can easily place waystones in my cities from code. Is something like that possible? Thanks!

The next release will have a placeWaystone(Level level, BlockPos pos, WaystoneStyle style) API method, but the waystone blocks can also lazily initialize themselves, so if you don't need any further access to the placed waystone data, you could also achieve this by manually placing two waystones:waystone (or mossy_waystone, sandy_waystone) blocks on top of each other and setting BlockStateProperties.DOUBLE_BLOCK_HALF to LOWER and UPPER respectively.