As you've noted, rather than is a bit of a strange beast. I can't offer a complete analysis, but it isn't grammatical to put a finite verb after than in the past tense. Rather, it must be either the uninflected base form or the present participle. The choice between them is partially controlled by the tense of the main verb.
He left rather than watch the movie.
? He left rather than watching the movie. (Sounds wrong to me)
*He left rather than watched the movie. (Always wrong)
From this article, I offer the following quote from the American Heritage Book of English Usage:
The phrase rather than consists of an
adverb and a conjunction and often
means “and not,” as in I decided to
skip lunch rather than eat in the
cafeteria again. It is grammatically
similar to sooner than in that it is
used with a “bare” infinitive—an
infinitive minus to: I would stay here
and eat flies sooner than go with
them.
Rather than can also be used with
nouns as a compound preposition
meaning “instead of”: I bought a
mountain bike rather than a ten-speed.
But some people object to this use,
insisting that than should be used
only as a conjunction. They therefore
object to constructions in which
rather than is followed by a gerund,
as in Rather than buying a new car, I
kept my old one. [Editor's note: these people are clearly insane. rather than + gerund is just fine, as is rather than + noun phrase.]
In some cases, however, rather than
can only be followed by a gerund and
not by a bare infinitive. If the main
verb of the sentence has a form that
does not allow parallel treatment of
the verb following rather than, you
cannot use a bare infinitive, and you
must use a gerund. This is often the
case when the main verb is in a past
tense or has a participle. Thus, you
must say The results of the study,
rather than ending (not end or ended)
the controversy, only added to it. If
the main verb was in the present tense
(add), you could use the bare
infinitive end.
Curiously, when the rather than
construction follows the main verb, it
can use other verb forms besides the
bare infinitive. Thus you can say The
results of the study added to the
controversy rather than ended it.
The overriding concern in all of this
should be to avoid faulty parallels,
as in sentences like Rather than buy a
new car, I have kept my old one and
Rather than take a cab, she is going
on foot.