Keng Leong Yeo
The "Ways To increase Bids" section (Page (9) cites no exceptions, and its been ruled that they can be used for Battle spaces as action spaces. While I'd think they cannot be used for this (instead of gold or armies), it could be argued they can--since its not disallowed in the Improve Tribal Relations section (Page 16) or anywhere else. But only gold or armies are mentioned as effective.
What's the official answer?
Bid tokens, the Han emperor token and popular support tokens can technically be used on the Improve Tribal Relations action spaces. However, they will serve no purpose since other players cannot bid on your state's Improve Tribal Relations action space. In other words, once you place a general on the Improve Tribal Relations action space, you are guaranteed of the Improve Tribal Relations action.
The only two ways to increase tribal relations levels is by gold or armies, as stated on page 16 of the rules:
Improve Tribal Relations
When resolving this action space, the player moves his tribe marker up his tribal relations track, towards the maximum of level 12. The number of spaces to move is based on the number of gold tokens or army units assigned with the general to the "improve tribal relations" action space:
• 0 gold tokens or 0 army units – 0 levels
• 1 gold token or 1 army unit – 1 level
• 2 gold tokens or 2 army units – 2 levels
Pay any assigned gold tokens to the general supply. Return any assigned army units to the player’s supply.