Its a great concept, but the issue is the implementation. We don't own the Towny plugin and therefore making modifications to the claiming features is more challenging. From doing some research a suggestion similar to this for Towny was made in 2011 and it was implemented in some capacity, but since then Towny has grown massively. Ideally if we can find a simple way of doing this then I'd be happy to look into it, but here's some issues right off the bat:
I've seen something similar to this on different servers, So you would get a infinite golden block that's only sent by the command that's used via the user. So when using a command which is possibly "/plot set block", it would say "Please place four golden blocks along the corners and the roofing." the system would recognize the room as owned and not use the whole chunk, however I'm not sure how they manage to do it. It doesn't use chunks, It just uses what the golden blocks based it off, And the plot would still have requirements like, Being in the town only, Must be being sold, Basic items, So basically a revamp of /plot. We can effectively show them what plot they are standing in as the golden block defines it's area, Breaking the use of normal chunks. You could still use chunks which would be added on to this. Also after use of /plot set block the golden block would disappear from your inventory, Only being used as a temporary block for fitting. Since you don't own the towny plugin which you stated, You could make a separate plugin to attach onto the server or disable the plot command for towny and implement your new plugin which uses both my technique and towny original plot usage. So basically, a normal player now would do "/plot claim" for when the plot has "[Sale: 0$]" now, the other player would like to build a house on top of the normal player, he would state "/plot set block" which would hand him a golden brick used up to infinite times but cannot be placed in a "spamming" manner. As he gets permission from normal player he builds his house, Places the golden brick on every known corner and the upper level should say "Not for sale" or "~Other Player" and the bottom should state "Not for sale" or "~Normal Player" this would help them not steal from each other. Here's a problem, How would you implement this kind of plugin; even though I'm not sure how, you can achieve this but I've seen it in some SMP's so you might possibly find a plugin for this.