I'm trying to make a throwable that places a specified Block on impact, and I'm trying to have it change to the appearance of the block specified. The ClientProxy render code doesn't seem to be usable for this, since it needs everything to be static. Is there any way to do this?

What do you mean by ClientProxy render code? You register a custom renderer for your entity class, and inside of that renderer you can do anything you want, including using information from your entity (provided you send that information to the client) to determine what texture to use.

 

If the texture does not change once the entity has spawned, implement IEntityAdditionalSpawnData in your entity class to add whatever information you need (such as a block ID) to the initial spawn packet.

 

If the texture CAN change after it has spawned, use DataWatcher to track the current block ID (or however else you want to do it) so that the client always knows what the current texture should be.