The usage of plus and other forms like plures, plura seems to be a little complicated and depend on the grammatical number.
Plus can be used as a singular noun or an adverb
The form plus looks like a singular neuter adjective in the nominative/accusative case.
However, what I've read is that this word in the singular is only used as an adverb (the nominative/...
As Cerberus points out, plus is an adjective and has therefore the gender and number and case of the main word.
There is also the corresponding adverb plus, which could be seen as the neuter accusative of the adjective.
The word magis is also an adverb, but not synonymous with plus.
I assume your question concerns the adverbial usage, but do bear in mind ...
The adjective plus means "more". If you want to say more than, you can either use the (often elliptical) conjunction quam, or an ablative of comparison.
[Ego] habeo plura capita quam homines [capita habent].
[Ego] habeo plura capita quam [ego habeo] caudas.
After quam, you would use the same case as the first element of the comparison, so ego ...
Here is some contextual information I was able to find (not a full answer--I hope the bounty will attract one):
What sources say about the dative ending
The dative ending -ū is well attested with non-neuter nouns, not just with neuter nouns:
An alternative dative ending -ū, which is normal for neuters like cornū 'to the horn', genū 'to the knee', often ...