This has been mentioned a few times by folks in Parity and as part of @joepetrowski’s Common Good Parachains roadmap:
It has been a long-standing idea within the Polkadot network for the on-chain Treasury to hold many assets, in addition to its native token. One, users making Treasury proposals may want to request funding in other assets besides DOT. In addition, the Polkadot stakeholder community as a whole may want Polkadot to acquire stakes in other assets. By providing the multi-asset support that Statemint specializes in, it can allow more options for Treasury use and management.
And then indirectly in relation to @xlc’s comments on XCM3:
If a multi-asset treasury is a way off - for the reasons you mention, then we can still experiment with governance ‘hacks’ of the system, with asset locks on parachains representing designated funds?