Store a cell format parameter in a Text field and use it is a cube view

Yeah CV Extenders only work when the CV is used to generate a report. If you're working with the pure CV, options are limited.

On the MF substitution variables, I checked and it looks like I fell foul of a change made a few years ago... they are not available in all properties anymore, they only work in row/column definitions by way of the XFMemberProperty function - which sadly does not execute in Conditional Formatting either.

So, with these constraints, unfortunately I don't think your strategy of storing formatting on account properties can work at all.

An alternative approach could be to use actual Parameters, storing generic formatting properties, and then switching between them if the member is in certain categories. E.g., let's assume you have a AssetFormat and a LiabilityFormat parameters; both are Literal Values, storing different formats (e.g. "Bold = True" for assets, "Bold = False" for liabilities). Your Conditional Formatting will look like this:

If (RowE1MemberName in XFBR(MyRule,GetAssets)) then
   |!AssetFormat!|
else
   |!LiabilityFormat!|
end if

At that point, the XFBR doesn't need any parameter, it just has to return a String containing the comma-separated list of Account members - which you can generate any way you want, including with lookups on Text properties (although in that case, you'll have to be careful with Scenario Type and Time...).

This is a relatively expensive strategy, since generating that list might take time, so you probably don't want to do it on each and every row; ideally you'd put that XFBR in another Parameter, so it's (typically) executed just once (when the CV opens) - ending up with "if (RowE1MemberName In |!MyGenList!|) then...".