We should instruct the PCF to open an investment account on behalf of the treasury. We should roll funds over time from the treasury into investments with yield and use the yield to buy DOT and top up the treasury on a regular basis. Treasuries were one thing that were brought up. Raw materials are normally pretty good too somewhat anti/counter-inflationary and with regular yield.
Unless we want to vote in an active manager all we can do is build a passive portfolio and rotate yields back into DOT.
Maybe we can buy some MSTR and get Saylor on board. 
but I am unfamiliar with any outlet that would allow the Polkadot DAO access to traditional asset classes in a decentralized manner
That is the entire purpose of PCF ( REF - WIKI )
A real world entity of, for and by the Polkadot Community. The PCF is “Web 2.5”, it’s a 1.0/2.0 compatible/compliant entity that acts as a bridge to 3.0. This entity would have no issues establishing a traditional investment holdings account.
Update:
After giving it more consideration and running the numbers I just don’t see how we can do this and for it to really provide any value short or long term. Even with amazing yields it’s likely to do more harm than good with the additional downward pressure that we’re already contending with from other DCAs mixed with lack of interest.
We need interest (and excitement) that we will only get by deploying services and applications that people want to use over traditional services because they’re better and/or cheaper. A few niche production ready use cases are really all we need to turn things around in a big way. With that increased interest it would make the numbers for ideas like this more practical.