I guess the orginal poster by now may well have sorted out a solution to their problem - but as a few comments:
Roger Woods (9/8/2009)
I did ask Sonifex at the last SBES (2008) if they intended to make a box to do this operation and they don't (currently) but would investigate the possibility, in fact I think I might have been the first person to ask about this as an option.
Obviously you only wish to switch one feed - but the sonifex red box which Simon suggested will work nicely with the PCI1750 card - and gives you ten switchable feeds. I've got just such a set-up at one station - we've yet to find 10 different feeds we want to switch, but did have at least 4 feeds on a regular basis, with level adjustment needed on each input, plus potentially other inputs, so worked out cheaper than multiple PSquared 'black boxes'.
I take the output of the Sonifex source selector into the line input of the Mia card in our automation PC - where it gets mixed in the sound card with the Myriad output to give me a combined 'automation' feed which becomes the 'studio 3' input on the Sonifex RB-OA3 studio switcher. (Note - make sure you run the source selector in 'mix' mode otherwise the last hardware line selected takes precedence and what Myriad thinks is switched on is not necessarily what actually is!)
I have avoided the external source input on the Studio Switcher for one simple reason - if one of the real studios are broadcasting - the Myriad automation PC may still be playing it's log (possibly even in 'preview' mode where time markers might not be hit) I wouldn't want extra audio getting injected into the broadcast chain outside of the control of the operator in the 'on-air' studio.
Going back to your situation, if you want to use the external input, I can't recall if the PSquared black box is stereo or mono, but assuming it is, basically it is just a couple of relays and associated electronics, assuming the box has a couple of DPDT type relays - if you weren't frightened to do your own mod you could switch one of the relays connection to the Normally Closed terminal rather than Normally Open where you could switch the news through one relay and the reverse logic for the 'mute' function on the studio switcher input.
If you are going to go down the 'build it' yourself front - take a look at Paul Blitz's page:
http://www.blitzfamily.eclipse.co.uk/paulblitz.co.uk/tech_tips/techtip/art_ind.htm
Incidentally the socket on the PCI1750 card is a 37 pin D-Type (not 25 as you state) and Maplin electronics no longer sell 37-way DTypes ("no one was buying them" they said) most annoying when you want one in a hurry!
--
Myriad user since 1998, now custodian of dozens of Myriad,Autotrack, Powerlog and Scoop boxes of many different flavours.
http://www.philedmonds.com