Since I started this thread, I thought I'd add a minor update and my thoughts on this whole topic.
Whether TurboTax ever supports the ability to claim 0%, the real issue is that tracking the CRA "proportional income attribution" for joint investment accounts is a real pain-in-the-butt. Especially if the relative proportions change across tax years. It causes work for you (tracking the changes and maintaining "proof"), and your work is rewarded by CRA with unnecessary queries/ follow ups/ re-assessments/ audits as proportions change.
As stated in my original response, I decided to "fix" this by openning pairs of joint accounts at each of the financial institutions we deal with (2 in my case, so 4 joint accounts). At each institution, we have one joint account with me as primary, and one with my wife as primary. It did not cost me anything (other than explaining to them why I was doing this). The financial institutions transferred the relevant investments in-kind so there were no capital gains/ losses triggered (in my original post above I thought I would have to sell and re-buy to do this). So was more-or-less effortless and solves this issue going forward.
My wife and I just direct investment money to the right account based on who is contributing it. All generated income is 100% attributed to the right person. There is no tracking/ accounting for income attribution I have to do for this. CRA is happy (hopefully) because all slips are reported to the correct account with 100% attribution and it never changes. TurboTax is happy as it automatically imports the correct slips from the correct CRA account and assigns it to the correct tax return.
So it would be good for TurboTax to allow the 0% setting ... but it may not solve the REAL problem, as long as CRA continues to flag "unusual" income attribution splits, and changes in the proportions across tax years. The paired joint accounts just avoid all of this.
