Never seen this. I would give ABIM a call to confirm. It's a minimum of 1 year of critical care medicine fellowship after any 2 or 3 year IM subspecialty fellowship. Not sure if hospice/palliative care qualifies. You can't do 1 year of sleep medicine and be board eligible for critical care medicine after a 1 year critical care fellowship.
Regardless of that, I think you probably want 2 years to be well trained. I know that many critical care medicine programs require nephrology/ID grads to do 2 years for procedure numbers - mainly intubations, bronchs, chest tubes. I would think they would do the same for a hospice doc.
As an aside - I wouldn't want my critical care physician to be a hospice physician. In many ways opposites.