I think in order to do that the ground would have to be prepared with a modified version of SRFI 144 for half-floats. I don't know which of its procedures, which result from a merger between R6RS and <math.h>, are still relevant, but someone sufficiently knowledgeable and/or experienced should take a look. For that matter, there is no SRFI for 32-bit floats either.
I have toyed around with the idea of a fully or partly table-driven implementation of 8-bit floats in which all the values are integral (see the second table at <
https://en.wikipedia.org/wiki/Minifloat>), but have put it aside as too low-priority for now.