Force index using the unused index columns having effect as order by for the result set?
There is no guarantee of any order without an explicit ORDER BY clause.
This is a fundamental aspect of SQL which deals with sets. Sets, by definition, have no order. An "Ordered Set" is no longer a set, it is a cursor.
Even if you happen to see rows come back in a certain order repeatedly due to indexes or execution plan operators, you should never assume it will always be that way.
I've seen many application fail due to making such assumptions.

I'm not sure what 'some sort of limitation' prevents you from using ORDER BY, but as a general rule, and since the order in this case is only for presentation purposes only, you should consider ordering the rows at the client side instead of at the database.
The client (or presentation tier) is responsible for presentation aspects such as order, fonts, colors, indentations etc.

Also note that your LIMIT clause can pick any 100 arbitrary rows without an ORDER BY.
Don't expect the LIMIT to pick up the first 100 rows based on some index order.

HTH.