I think it's fine to include the text of the Boost license in a NOTICE.txt file. Usually that's what I do when there are differently-licensed third-party projects which we adapt or build upon. (Of course: IANAL)
According to tl;drLegal:
You must include the original copyright and this license in software
And from the full license:
The copyright notices in the Software and this entire statement, including the above license grant, this restriction and the following disclaimer, must be included in all copies of the Software, in whole or in part, and all derivative works of the Software
Whereas the BSD-2 license says:
Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
And the MIT license says:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
So the Boost license appears a little more stringent to me ("must be included in all copies of the Software, in whole or in part"). If you want to be totally thorough, you could put it in the copyright header of every single source file, but doing that well is a real pain. So personally I would vote for the
NOTICE.txt approach, the same as I personally would do with BSD or MIT base code. The pragmatic question is really whether anyone will get sued, and if you include the notice there, I highly doubt there will be any problems or misunderstanding.
Actually, I do sometimes also put the license blurb near the top of source files which were specifically adapted—since usually, the project only has a few files adapted in that manner, versus most other files written from scratch. If you do that (and I think it would be a good idea), I would suggest putting the blurb below your normal blanket license, so that the License Maven plugin can still easily do its job, without disrupting the Boost license portion beneath.