Unfortunately no, there aren't any natures that raise / lower HP. The problem is not in the conception of those extra natures (including matches with, say, +Atk - HP), but rather the coding of the game.
For those or you who know what PIDs are, you'll be able to understand this. PIDs are generated for a Pokemon upon meeting them (or an egg being laid), based on random numbers, and contain the information on their natures, genders, abilities, and other unique aspects (Spinda's spots, Unown form, etc.). However, the data necessary to determine the nature is limited to 25 possible spots, to keep each nature able to show up with an equal chance (25 / 100 = 4% probability for a nature to show up). If Game Freak were to add more natures, they would have to come out with 25 new natures to make it an even 50 (50 / 100 = 2% probability) to give each nature an equal possibility to show up.
Again, if Game Freak were to add natures for HP +/-, they'd need to think of more natures that skew the stats in other ways (HP+ only, HP-andAtk-, etc.), but the numbers would be bad:
Currently:
5 stats+ x 5 stats- = 25 natures, 5 of which are neutral (DIVISIBLE INTO 100)
Adding HP to current standards:
6 stats+ x 6 stats- = 36 natures, 6 of which are neutral
Considering single boost/decrease stats:
6 stats+ + 6 stats- = 12 natures, each one affecting only one stat (e.g., HP+ = Strong nature)
Total of above: 48 natures; 2 natures remaining for an even 50.
Considering double boosts / double decreases with current natures:
5 stats x 5 stats x 2 directions (boost/decrease) = 50, 10 of which doubly affect a stat, 20 of which are neutral (10 outcomes, each repeated twice => UNEVEN PROBABILITY)
Adding HP to double boosts / double decreases:
6 stats x 6 stats x 2 directions (boost/decrease) = 72, 12 of which doubly affect a stat
Hence, 25 natures was the best they could do for now.
Sorry for the long answer, but I'm sure somebody out there knows what I'm talking about.
