>>I’ve set up an elaborate workflow to undo everything that Close Lost kicked off. Is that how you’d handle that?
“elaborate workflow” -- hmmm. Sounds scary.
- see The HubSpot Workflow Butterfly Effect
Without knowing “everything that Close Lost kicked off”. (important)
We never recommend moving Lifecycle Stage backwards because it disturbs conversion stats being auto-recorded in HubSpot. Instead, we use the COMBINATION of Lifecycle Stage = Opportnity AND Lead Status = Recycle Lead to initiate the lead RE-ENGAGEMENT process (which is different than the original engagement workflow).
We basically TREAT them like SQLs without ever changing their Lifecycle Stage back to SQL, thereby preserving the original MQL-to-SQL and SQL-to-Opportunity conversion stats being auto-recorded in HubSpot.
In most cases, this simply means separate campaigns from the original SQL-Opportunity flow. Meaning, separate early, mid, and late-stage engagement drip email campaigns now called RE-ENGAGEMENT campaigns (typically with tighter engagement constraints) -- separate workflows, and, in some cases, even separate LPs. (highly portal-specific)
Ok, maybe “simply” was an over-simplification. 
More work, but worth it to those who appreciate the escalating costs of lead acquisition and engagement. Admittedly, our views on closed lost deals are skewed strongly in favor of re-engagement because, as business owners, we actually foot the bill for lead acquisition in all its forms …
That being said, no amount of re-engagement is worth sacrificing original conversion data without which we have no real idea how effective our marketing and sales efforts are. IMHO -- sorta like calling oneself ‘data-driven’, but saying “to he11 with the data”. 
Hope that helps.