The difference is in the degree of probability or likelihood that someone might get to go to Japan.
ようなら expresses a fairly good chance and the speaker would usually have in mind a specific time frame for the event to occur. = "If it looks like I could go to Japan (e.g. next year), ~~~."
ものなら implies a very low possibility. = "If I ever get to go to Japan, ~~~."
The difference between those two is nearly nonexistent, at least in terms of the degree of probability. I could not, off the top of my head, think of a situation where the two could not be used interchangeably.
It just sounds more refined or mature if one adds the よう there than not.
～ものなら（ば）seems like it could be similar to とすると to express a hypothetical or uncertain assumption about something but ～ものなら（ば） may be more natural in the case of a personal wish or circumstance that is difficult to fulfill (If it is possible that/if only I could....)