I think a BIP is a good idea, but rather than making such a specific proposal as "Let's use bit 4 to indicate communication of thin blocks," how about a more general one like "Let's use bit(s?) 4(-5?) as user-agent specific service bits so that if you customize your user-agent string, you can use them for whatever you want"? That way, other clients can choose to follow suit by saying so, or simply recognize the meaning (or lack thereof) of those bits based on the user-agent setting.  This relieves future development from the burden of agreeing on where to put what, and allows time and utility to show when such a user-agent-specific service bit should be moved into the protocol section of service bits.

PS I am not well versed in the creation of standards, but the reservation of digital real estate for self-identified customization (bits, bytes, or whatever that will never be used by the standard) such as what I'm proposing seems like something that probably has a standard name.  "Public provisioning" or something like that?