One doesn't ordinarily say
- *I'm being there tomorrow.
to indicate one's future location. True.
On the other hand, one doesn't ordinarily say
to indicate one's present location, either.
The problem is not with the future, nor with the rule about using progressive for future, though it's a stupid rule if it doesn't tell you that the progressive (or continuous) construction does not apply to stative predicates. And locatives are stative. Similar problems occur with other stative predicates:
- *He's being tall.
- *It's being warm today.
- *He's owning that house.
The progressive construction is for Active predicates -- verbs, mostly, like go, run, sit, rent, compile, write, ..., but also behavioral adjectives like dishonest, impolite, rude, ... You can say He's being rude but not *He's being tall. So, for active predicates, you can use the progressive to indicate future time, under the appropriate circumstances.
Executive summary: Don't take grammar rules too seriously. Most of the ones in books are BS.