Custom Validation Before Saving Cube View Data

That's what Confirmation Rules are for, in traditional OneStream: you let users submit what they want, then you run a rule on the resulting data (either with a Confirm step in workflows, or by running BRApi.DataQuality.Process.ExecuteConfirmation in a rule).

You could approximate your requirements by having some sort of widget that signals the status (like a red light/green light), and setting its appearance with a Server Task triggered on save (you will probably need to refresh its container dashboard to see it change) that runs confirmation rules or your own checks. The data would be saved, but the user would know it is wrong. Having the check as a Confirmation Rule would also allow you to reuse it in other validation processes.

If you want something more, you need to look at custom forms or the new Dynamic Grid.