Perplexed in Peoria wrote:

 So, I'm curious
whether there is any such thing as a fusion breeder reactor
in which excess D is converted (perhaps endothermically) into
something more useful?

Deuterium can be fused with itself.

D + D --> T + p
D + D --> 3He + n

One current alternative fusion design, the dipole fusion reactor,
that seems well-suited to use in space (since it has a large plasma
volume, and a levitated coil to produce the dipole), may also
be particularly suitable for burning deuterium in the
'3He catalyzed D-D cycle'.

https://wormhole.ucllnl.org/ICC2003/dbms/posters/poster1.pdf

The 3He here comes entirely from the DD reactions (either the
second reaction, or the first one followed by removal of T, which
is allowed to decay to 3He.)

Of course, even He3 is practically inexhaustible if you can
extract it from gas giant atmospheres.  But getting it out
then to the Oort cloud thru trade seems very problematic.

Smallish planets (mars sized?) far from any star could
retain 3He in their atmospheres, since the temperature at
the exobase could be quite low.  These planets might
accumulate helium (and hydrogen?) over billions of years,
particularly when passing through dense gas clouds.

    Paul