Use UART Tx as digital output?
Would it be possible to send a 5 volt "true" signal form the Tx of a UART dongle? As if it were a digital output?


Conditional yes.

(1) The module you linked to has TTL output levels.
Assume it uses +5V = high and 0V = low (I see it has 3V3 and 5V mentioned on its PCB).  

This means that RXD will be at 5V in the idle state, a start bit will be 0V, data bits will be +5 for logic 1 and 0V for logic 0 and sytop bits will be 5V.

When data is not being sent V_RXD will be 5V.

If you send a $00 signal (00000000 binary) the UART will output
 start + 8xdata + stop = 0 00000000 1
 The average DC level for this signal will be 1/10 x 5V = 0.5V.  

If you resend successive $00 bytes at maximum channel speed the output may be as low as 0.5V DC when suitably filtered. Depending on hardware and the USB interface and PC configuration and the phase of the moon you MAY get interuptions between btes and see occasional 5V signal level.

If you add a very simple RC + diode filter such that a  RDX low discharges the cap immediately but charge up is via the resistor you can get low output during periods that the TXD line is high for limited periods.
 If $00 bytes are able to be sent end-to-end with no breaks an RC+D filter can have a time constant of only a bit time or so. If longer highs occur you need a longer time constant.  

Whether an RC+D filter suits your needs depends on the data rate you want and system behaviour. If this is to control something that needs slow response - say  1 second or longer between changes and slow low to high change is acceptable, then this could work well for you.



(2) External UART: 

A better method is to add an external  UART at the receiving end and decode the serial  signal to a parallel word. This gives you 8 bits of on off.



(3) If you want something already built that does what you want buy a USB to parallel port (printer port) converter - this gives 8 (or more) lines of on/off control from USB. Many on ebay. Here is one for $US2.66 buy now withe free shipping !!!!!!!!!!!!!!!!! . Many more similar. Some people have commented that this may have printer functionality but not parallel port functionality. If so it should still be usable. At worst you may need to latch the data at "print time". 





Added:

Indented text added as a result of a comment by CS:


You specifically mentioned using the TX line, which is what I initially responded to.
Others have mentioned using flow control lines (which did not visibly appear to be available on your device and were not mentioned in the ad)and some suggested buying a converter which had these signals.

I recently spotted a USB-TTL serial converter on a local auction site and noted that the holes along the sides of the board had flow control labels. As your unit has similar holes it MAY also have similar functionality. So ...


Look at the bottom of your USB to TTL converter - it may have RTS/CTS and perhaps other signals available. If so, one or more of these can be controlled as an output. 

See photo below. This is your unit. Note holes marked by red lines.



The photo below is a unit sold on a local auction site.
 Of the 4 lines marked by arrows, 2 will be outputs and two will be inputs. These are controllable as part of a COM port. Yours may have these lines active or they may be disabled. Do you have a data sheet or detailed model number?