You're on to something there Tony. Surely there's someone willing to store your bike in Mendoza at least.
There is a possible solution to leave the bike for longer than the 90 days in Chile but not sure if it works. You can get an extension, or Prórroga, by filling out a form for the Aduana, best bet is in Santiago Airport, with photocopies of your docs/licence/title. You need a reason they deem as valid to extend it and you need to do it at or close to the expiration of your TVIP. You can definitely get an extra 90 days, unsure if you can get more.
Assuming you can't get more than 90 days and for the 6 mth departure question, you could email the Aduana explaining your situation, close to the expiration of your TVIP when you're out of the country, asking them for an extension with justification and see what they say. This one is risky though of course without ageeing it with them before you leave...
Otherwise, private aduana house suggestion could work. There is an Aduana office at Beauchef 1489 in Santiago, near Parque O'Higgins with plenty of cars impounded. Not sure about bikes there and not sure if that offers a solution to appease the Aduana by leaving it there.
Christian at Hostal Casa Matte in Stgo is getting quite chummy with the Aduana so he might have an idea if you contact him. You can contact him on Facebook.
If you're outside of Stgo, not sure what you can do...