A molecular marker for global protein synthesis
The answer will not satisfy you. The truth is no protein marker will do.

On one hand you seem to be looking at some individual, representative protein, which you could measure by immunoblot. But most proteins are in scant concentrations, just as many as required for their job, and go up and down depending on their specific roles, rather than on general metabolic trends. Even if some general control mechanism increases protein synthesis across the board, in a cell proteins are 20% actin, 20% tubulin, 0.0001% 4E-BP and 0.0001% p70-S6K (made-up numbers for the sake of example), any 1% total protein increase you'd like to measure would be 200,000 easier to detect in tubulin than in p70-R6K. In order to be a marker of general control of protein synthesis, the marker protein you'd want to measure should be a plentiful protein, with little to no specific control.

On the other hand, those are the characteristics of a house-keeping gene. So you won't be able to normalize such a "representative" blot! Normalizing house-keeping by house-keeping, you will have pretty much no change.

Embrace labeling experiments. Even there, the right normalization is debatable. If anything, the fact that everyone does those, rather than looking at protein X, should be an indication of how much you will struggle to prove points about protein synthesis rate using anything else.