I don't know about amateur built transmitters at that power output. I have a feeling both the FAA and FCC would have something to say about that. You mention getting it certified, and that can be a heck of a process. I'll defer to others with more knowledge of this feasibility.
On the other hand...
ADS-B *in* capability is very much within the grasp of the hobbyist. I've been looking at 1090ES recently as there seems to be a more active hobbyist community listening in on those transmissions. The basics of it are a low noise amplifier, 1-2 filters (1090 MHz SAW filters are available from several places), and most often a logarithmic amplifier as an amp/detector. The output can be sampled with an analog-to-digital converter or a comparator driving logic level inputs into a microcontroller which then does the very simple decoding. I assume a 978 MHz (UAT) receiver would be more or less the same. From there, it can be as easy as plugging it into a laptop for display.
Some websites of interest:
http://miniadsb.web99.de/ (these guys sell a kit of a complete RF front end for 1090ES reception)
And finally, the holy grail for information about both UAT and 1090ES, hidden under the guise of boring, bureaucratic technical meetings. See working group 3 and 5 and click their most recent meetings. Draft versions of DO-260B (1090ES ADS-B standard) and DO-282B (UAT standard) are available. They are the most recent draft versions, but I can't imagine much of the technical information changed before publication. For what it's worth, these documents go for $500-$700.