I think it’s a great idea and an important feature too.
I don’t think it’d be possible to be done using MetaMask as mentioned only because the funds are in Aave’s contracts so there would be no way for MetaMask to really make that swap without you withdrawing first. MetaMask’s swap feature is essentially a DEX aggregator (DEXAG, 1inch, etc.) built in, so doesn’t bring new possibilities per say mostly just convenience.
All of that said, collateral swapping/trading is planned to be supported in Aave V2, which is explained more in this blog post here: https://medium.com/aave/aave-v2-the-seamless-finance-d52075d97a70