No, in closed shop contracts, this is expressly prohibited. Violating this and negotiating with your employer directly could be considered grounds for termination of union membership (ie, termination of employment). The employer would also face consequences as well.

> whatever has traditionally been agreed upon for steelworkers etc. may not be the best model for tech workers.

That's true - unfortunately, the NLRA (the law which regulates union operations) is very rigid, and it does not provide different stipulations for different industries.

There's a reason that virtually all NLRA-regulated unions in each state enact the same corporate policies for membership, the same contract structures, etc. - those are the ones which turn out to provide a stable (in the literal sense) balance of power under the laws.

It's very unlikely that an NLRA-regulated union in the tech industry would operate differently, in the long run, from the NLRA-regulated unions in every other industry.