The Nooelec Ham-it-up already has a circuit in it which you might be able to use in instantaneously switching it out of the circuit when you transmit.
Combined with a relay it would almost certainly be adequate.
I would go to the Skyworks site and look up whatever device they use. http://www.skyworksinc.com/products_switches.aspx
They are very popular and I'm sure very good. Better than mechanical switches as far as loss at high frequencies.
The Ham-it-Up presents very little loss at VHF/UHF when its turned off.
The schematics for the entire Nooelect device are online.
Was looking for some data on Pin Diodes yesterday and found this: http://www.microsemi.com/sites/default/files/micnotes/701.pdf
"PIN Diode Fundamentals
A PIN diode is a semiconductor
device that operates as a variable
resistor at RF and microwave
frequencies. The resistance value of
the PIN diode is determined only by
the forward biased dc current. In
switch and attenuator applications,
the PIN diode should ideally control
the RF signal level without
introducing distortion which might
change the shape of the RF signal.
An important additional feature of the
PIN diode is its ability to control large
RF signals while using much smaller
levels of dc excitation."
Since it doesn't have an LNA, I dont see why the Hamitup and similar simple converters wouldn't be able to be used in reverse for transmitting.