This isn't much of an answer, but as the question does specifically ask about whether there is a difference between the light novels and the other versions, here are my findings:
In Yen Press's English translation of A Certain Magical Index: Volume 3, which hand of Touma's that Misaka 10032 is holding to her chest is unspecified. Here's the quote:
Lastly, he noticed Little Misaka's hands embracing one of his.
It didn't really matter, but she was holding both of her hands to her chest...
Furthermore, it also isn't specified which side of the bed Misaka 10032 is sitting on:
After all, the only person in the room was Little Misaka, quietly sitting on a chair at the side of the bed-
Misaka 10032's explanation for why she was holding Touma's hand to her chest are same in the manga and light novel: she says she's measuring his brain waves and pulse via bioelectricity. Neither offers any more insight than that.
From this, one might find it reasonable to conclude that it was indeed an error introduced by the anime and that by the ambiguity in the light novels, we should conclude it was Touma's left hand because it would be impossible for Misaka 10032 to read his bioelectricity from his right.