The answer here is a resounding no.

From 14CFR61.31(i) ... no person may act as pilot in command of a
tailwheel airplane unless that person has ... received an endorsement
in the person's logbook from an authorized instructor who found the
person proficient in the operation of a tailwheel airplane.

Note that this is not limited to takeoff and landing. Therefore, the
CFII who lacks a tailwheel endorsement may not serve as PIC in a
tailwheel airplane at all. The student, who lacks an instrument
rating, may not serve as PIC under IFR.

As a practical matter, this would work fine until the day the weather
was really bad. When ceilings are low (say less than 500 ft) and
visibilities are bad (less than 2 miles) it often becomes necessary or
highly desirable to land with a quartering tailwind, on wet pavement,
with insufficient visibility for precise aircraft control without
reference to instruments, or all the above. In such conditions, one
really needs instrument experience AND tailwheel experience to
successfully make the transition from instrument flight to visual
flight and a safe landing. The process is not inherently different
for a tailwheel airplane than it is for a tri-gear airplane, but there
are things you can get by with in a trike that will put you in the
weeds (or worse) in a taildragger.

Michael

(Doug) wrote
Client is an instrument rated pilot with his own tailwheel, instrument
equipped airplane. Client is current on tailwheel, but lacks currency
in IFR.

Instructor is CFII and current, but lacks a tailwheel endorsement.

Client wants to take off in VFR, and open an IFR flight plan (on the
instructors qualifications), and fly the IFR flight plan.

Can the instructor file an IFR flight plan and open it in the air
(after takeoff) fly the IFR flight plan and cancel IFR before landing
with the agreement that the client is PIC for takeoff and landing?

It seems to me that the question is "is it necessary to be acting as
PIC in order to open and accept an IFR clearance?" because the CFII
cannot act as PIC in a tailwheel aircraft, yet he does have "catagory
and class" ratings necessary to accept an IFR clearance. But I will
have to admit, I really don't know.