I have hunted high and low for the answer to this but haven’t been able to find any amending SI.
However, looking at the IR website for tax credits shows that to get the disability element of WTC a person must, unsurprisingly, be disabled. There is a comprehensive list of what constitutes disability thus making a person eligible for the disability element.
Many of the criteria are identical to some of the other subparas of Reg 3 e.g. DLA, AA, SDA etc.
Some of the criteria are very loose such as Incapacity Benefit or an invalid carriage under the Invalid Vehicle Scheme.
Because of the far wider definition of disabled under WTC rules, and the fact that most people will be getting one of the more traditional disability benefits already listed as qualifying, it appears that just getting the disability element of WTC does not qualify a person for SMI disregard.
However, if you can establish why a person gets disability element that might, in itself, qualify a person to get SMI disregard.
Hope that helps.