There are two parts to this answer.
Firstly what you ask can be done. It is a bit complex and I will not provide the solution at this point.
The second part is that what you ask may not be what you really need and so I will address this. The most common time people want to weight over dimension members is when the actual is really a ratio or percentage. e.g. tonnes per worker. Imagine A is a big unit producing 60 tonnes and B is a small unit producing 40 tonnes. You want actuals to be tonnes per worker. A has 2 tomes per worker and B has 3 tonnes per worker. The parent cannot be 5 tones per worker which is what would usually happen when you consolidate. Nor can the parent be 2.5 tonnes per worker (average of A and B).
The tonnes per worker actually comprises two measures, tonnes and workers. A has 60 tonnes, 30 workers and a score of 2. B has 40 tonnes, 13.33 workers and a score of 3. The parent has 100 tonnes, 43.33 workers and a score of 2.308.
Is this is what you want to achieve? If so assume you have a KPI TonnesPerWorker.
Add four new measures TonnesAct, TonnesTar, WorkersAct and WorkersTar. These will never be displayed in your scorecard. Load data to these measures at the base level and consolidate.
Now change TonnesPerWorkerAct to a virtual measure as TonnesAct / WorkersAct.
Now change TonnesPerWorkerTar to a virtual measure as TonnesTar / WorkersTar
Change TonnesPerWorkerTarDev to (TonnesAct / WorkersAct) % (TonnesTar / WorkersTar).
Finally if this is what you want to achieve there is a further refinement tat allows you to load the target as a ratio (TonnesPerWorker) and actuals as the two base measures (tonnes and actuals). I can expand on that but first let's check whether I have correctly understood your requirement.
Cliff