This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

The goal of the Federal Government is to balance cost and best performance during an acquisition.  The best and most efficient way to do this is through direct discussions with industry.  Before the government releases a formal Request for Proposal (RFP), it is reasonable to say the government MUST have industry engagement.  The key to this is that all responsible parties are aware of the opportunity to engage.  We are not required to engage with all, just those that specifically ask.  If a company that we believe has something to offer does not contact us, we can contact the company.  If the market is too big for the government to speak with each offeror, we should set up an Industry Day through RFI, FedBizOps, etc. and invite interested parties to attend (most likely done virtually in the present environment).

We need to make an observation given your question.  You stated, "(...i.e. after white papers/ pre-proposals are submitted but before final proposals)"  Once the formal selection process begins all communications flow into and out of the government through the contracting officer.  We are not sure, but in the context of your question, the "pre-proposal"  is not a pre-proposal, but the proposal before the process has reached the time for Final Proposal Revisions (FPRs).  Again, once industry has made its formal submissions, industry contact if through the contracting officer.