Powell has over time for me come across as something of a paper tiger, a potentially strong candidate whose real strengths were based on people's perceptions of him rather than his actual views. The moment he hits the campaign trail those poll numbers were going to crash as Republican voters enter the "respect him, but no longer my choice" category. It doesn't help that the issue of abortion was a relatively high-profile issue at the time, and Powell's pro-choice position would have alienated a whole swathe of voters who had become invested in that fight by the middle of 1995.
Howard Phillips would have been elated certainly given he tried to get Buchanan to run as the Taxpayers nominee in both '92 and '96, and I'd say there is certainly a good chance he would should Powell somehow capture the nomination and decide to run with someone like Arlen Specter. Whether it would somehow serve as the kind of release valve Thurmond's campaign did for Harry Truman back in '48 I honestly don't know, that being dependent on how loyal Republicans would be to the ticket after the primary fight and whether Powell can expand his appeal; if he is losing the Conservatives to Buchanan or they are staying home, that just means he needs that many more Moderates to make up the margins against Clinton.
There isn't much chance of Colin Powell making a break into the African American demographic. Sure you might see a spike in support in terms of Powell being the first African-American nominee, but I find it hard to believe it would be that substantial as on issues that African-Americans care about Powell would be pressured to take a relatively Republican viewpoint, and as you said a lot of the more influential African-American figures would be making the rounds and working to make sure the community stays loyal to Clinton and the Democratic Party.