> The fact that the rules explicitly disallow lending, but not any other deals, could point to other deals being allowed.

The rules explicitly allow particular kinds of deals and not others:

    Unimproved properties, railroads and
    utilities (but not buildings) may be sold to any player as a private
    transaction for any amount the owner can get; however, no property
    can be sold to another player if buildings are standing on any
    properties of that color-group.