Zorro: As far as grammar is concerned, нельзя can definitely be used with было or будет to show past or future. However, I'm not 100% sure if it's the best word to use in your example sentence about restrictions on Jews (possibly a stronger term like запрещено, "forbidden," would sound better). But I'd ask the native speakers about the word choice.

P.S. To me, Нельзя было еврейям seems a little ambiguous, and could be interpreted as meaning (for example) that the Jews themselves considered it improper or "unkosher" to work in government jobs. But the actual meaning, of course, is that the government banned Jews from even applying for jobs.