Samantha123:
But I personally believe that all cultures have their way of showing respect, not just the Yoruba tribe.

How can I get a Yoruba person to greet me by lying flat on the floor?

It's the yoruba way.

As for your question, if anyone sees you deserving of such, he or she would. Respect is earned.

Samantha123:
I once deliberately asked my friend to do the greeting for me while standing on a dirty floor.

He was nicely dressed, so I wanted him to get dirty in the name of showing respect..

He refused and said they only do those type greetings towards elders, how true is it?

You only prostrate flat to show respect to elders when you're trying to apologise for something you did wrong. Or you're being reprimanded for a wrong deed(mostly in a gathering of more than two elders).

You can causally greet an elder by bending half knee as woman or allow your knee to touch the ground. You can't be in the presence of a king and bend half knee. You go all the way.
And if you're outside (where your clothes may likely get dirty) you simply bend and touch the ground with one or two fingers. Even the elders won't allow you lie flat in such environment.

Respect is a very big deal in Yoruba land. Mastering the language of Respect will open doors for you as far as a yoruba man/woman is concerned.

Samantha123:
I once deliberately asked my friend to do the greeting for me while standing on a dirty floor.

He was nicely dressed, so I wanted him to get dirty in the name of showing respect..

He refused and said they only do those type greetings towards elders, how true is it?

You only prostrate flat to show respect to elders when you're trying to apologise for something you did wrong. Or you're being reprimanded for a wrong deed(mostly in a gathering of more than two elders).

You can causally greet an elder by bending half knee as woman or allow your knee to touch the ground. You can't be in the presence of a king and bend half knee. You go all the way.
And if you're outside (where your clothes may likely get dirty) you simply bend and touch the ground with one or two fingers. Even the elders won't allow you lie flat in such environment.

Respect is a very big deal in Yoruba land. Mastering the language of Respect will open doors for you as long as a yoruba man/woman is concerned.