Sideslipping not recommended on my aircraft
And missing the only emergency landing field in the neigborhood in an engine-out scenario is?
I don't know what sort of airplane you fly, but as far as I know, the main reason for sideslipping not to be recommended (or forbidden outright) is typically that either the rudder and/or the fuselage, in a sideslip, blanks part of the elevator, so elevator authority is reduced. At low speed, possibly to the point of losing authority altogether. But if you sideslip at best glide speed or a little above that, in the given scenario, it might be your safest option.
In any case, if you need to lose height to make the field, remaining at "best glide" speed is NOT what you want. If you slow down to just a few knots above stall speed (in the buffet, but not into the full stall) or speed up to a higher speed, by definition your glide path will be steeper than at "best glide" speed. Although speeding up means that you need to lose that energy on final again. Granted - I would leave the aircraft trimmed at best glide speed, and force it onto another speed with the stick or yoke.