In short, yes. There is already an intermediate token instrument at Grosmont, and it would be possible in theory to change it to a short section token. However it would need the signalling of the connection to the NYMR reviewing, as the hypothetical consequences of a SPAD at Grosmont are dramatically more serious now that trains can approach in both directions on the single line. Not a show-stopper by any means, but an additional cost.
Staffing of Signalling used by NR trains under contract to the NYMR is extremely unlikely to be acceptable to either party. The financial implications for the NYMR and the Operational risk to NR would be very serious. Far more likely would be for the NYMR to contribute towards the additional signalling shift for an NR signaller, as railtours do when they require boxes to be switched in outside of normal operating hours. However, see below:
Unless I've massively missed something, the line is No Signaller Token Remote, not RETB. NSTR has physical token machines but is released from (in this case) Nunthorpe controlling the entire line. I don't know whether any NSTR line has 'long section' working but there is, in theory, no reason why you could not have Glaisdale-Whitby as a long section with Glaisdale-Grosmont and Grosmont-Whitby as short sections. Some method of control would be needed to ensure the Glaisdale-Grosmont token was not issued to a train that would then end up in a mexican stand-off.
If RETB was introduced on the line, it would be far, far simpler to just give the NYMR a portable RETB unit and have Long/Short section working between Glaisdale, Grosmont and Whitby than to have an actual block post.