It's really not all that hard to estimate what the degradation will be.
Use your favorite inductance formula or program to find the inductance
of the whole coil, and then find the inductance of the unshorted part
if it were standing alone, and the inductance of the shorted part if it
were standing alone. From those three values you can find the mutual
inductance between the coils, and the coefficient of coupling. You'll
probably find it's much lower than you had guessed. The result is that
shorting the unused portion really doesn't have that much effect on
either the inductance or the Q of the remaining portion.

For example, an inductance program I use tells me that 20 turns of
8AWG wire 2" inside diameter helix, spaced with a pitch of two wire
diameters, will give me a coil just over 4" long, 8.705uH, unloaded Q
around 860 at 10MHz. (There's some question if the unloaded Q would
actually be that high, but that's another topic.) If I use 10 turns of
that same winding geometry, I get 3.637uH and Qu about 720 at 10MHz.
The mutual inductance between the two 10-turn halves of the 20-turn
coil is then 0.5*(L(20t)-2*L(10t)) = .716uH, and the coefficient of
coupling is mutual inductance divided by sqrt(L(10t)*L(10t)), or .197.
If you leave the unused portion of the coil open, the inductance of the
10t part you're using is, of course, 3.637uH. If you short out the
unused portion, the inductance of the used part drops to about 3.496uH,
which is only about 3.9 percent. The Q drops by about twice that
percentage. But if the LOADED Q of the network is around 10, and the
unloaded Q is up around 500, then only 1/50 of the input power is
dissipated in the coil resistance, and a change of even 10 percent on
the unloaded Q will result in a change of 10 percent in the power
dissipated in the coil...so in a 1kW system, the power dissipation in
the coil might go from 20 watts to 22 watts, in this example.

In summary, the assumption that the (unloaded) Q drops drastically is
incorrect for practical coils commonly used in transmitter tank
circuits. But it does suggest that you should understand what the
coupling is, and insure that the shorted part of the coil really isn't
coupled too strongly with the unshorted part: if the coil is very
short compared with its diameter, you'd get into trouble shorting
turns.

Cheers,
Tom