A little late to the party but you can now do this with the dotnet-sdk-versions tap.
You can find it here: https://github.com/isen-ng/homebrew-dotnet-sdk-versions
brew tap isen-ng/dotnet-sdk-versions
brew install --cask dotnet-sdk2-2-100
dotnet --list-sdks
Disclaimer: I wrote the tap 2 days ago