Don't beat me up over this, but could an old school antenna with "fine tuning" be used in the place of that $200 plus ATU that I.AM is selling?
has discontinued that ATU about a year or so back.
You can use whatever you want as an antenna, as long as it meets Part 15 regulations. It would void the FCC certification of the device, however.
So if I use any "Wild Card" antenna, that antenna itself could push my coverage area over the FCC's written 200 feet legal limit, or would the antenna itself be an improper modification?
Now if the antenna itself would be an improper modification, would there be an antenna (with tuner) that could substitute as a proper modification that is not the stock ATU that used to be sold along with the transmitter?
There is no range limit for Part15.219, which is what the Talking House is certified under. The salient restrictions are antenna+ground+feedline less than 3 meters, and 100mw maximum input to the final stage of the transmitter. That 200 feet limit was in a technical bulletin, and even the FCC admits that it's not accurate.
The Talking House certification with the stock ATU is actually a special case. Somehow the manufacturer managed to get the unit (with stock ATU) certified, by arguing that the ATU was the final stage of the transmitter. In doing so, the FCC ignored its own rules, and that was a one of. Talking Sign, as an example, could not get their equivalent transmitter and ATU certified. So if you are concerned with strict legality, you should use (if you can find one) the stock ATU. No other ATU would do (as the FCC could consider the coax connection between the ATU and the transmitter as a feedline, and that would violate Part 15.219 rules.
I hope that is clear.
You can use whatever antenna you want on the Talking House as long as you mount the transmitter at the bottom of the antenna (so that there's no feedline), do not elevate the installation (so that you don't have a long ground wire), and keep the antenna length to 3 meters minus the length of your ground wire.
Concerning the Talking House / IAm ATU.. Although they appear to have been discontinued.. As most of you probally know ISS utilize the Talking House transmitters for their "InfOspot" (part 15). What I find particuarly intersting is that the appear to currently utilize a external ATU with the InfOspot. http://www.theradiosource.com/products/infospot.htm
Case Study: http://www.theradiosource.com/articles/case-study-savannah-nwr.htm
The Talking House is enclosed in the lower section:
and the ATU and antenna is seperate and positioned above the transmitter: 
Bill Baker at ISS told me they discontinued the InfoSpot using the ATU. Instead they put the Talking House up the pole using the internal tuner and standard CB type 102" whip. The audio equipment is below for access as before.
I'm surprised to see that they put the original setup with ATU back online. The only constant is change.
This is the product description:
Transmitter/Tuner
FCC certified.
Operating under FCC Rules Part 15, Section 15.219.
Frequencies 530-1700 kHz, selectable.
Adjustable tuner circuit matches transmitter to antenna for maximum range.
Transmitter/tuner is housed in lockable weatherproof cabinet with integral 3 meter whip antenna & mount.
Oh, I see that case study is from an install 5 years ago.. I guess that explains the ATU.
The product description page also says: ".. range of up to 1/2 mile even when mounted at a modest height of 20 feet above ground." -- I thought that odd for two reasons;
1. it's indicating a 20 foot ground lead, and
2. in most cases a 30ft antenna system would provide much more than 1/2 mile range.
Another thing that puzzles me is "Separation distance from component cabinet to transmitter/tuner cabinet: minimum 1000 feet (wire not included unless specified)." -- What?? Why?
I would have to guess that's a typo. Probably meant maximum separation not minimum.