Sun in Libra and Sun (of any sign) in 7th are both concerned with relationships. Under the alphabet system of astrology, it seems that both are similar and equivalent. But there’s a clear difference as most of you would know.
A person with Sun in Libra suggest that his/her identity, life purpose, creativity, vitality, heroic journey, etc involve themes such as one-to-one relationships, beauty, balance and harmony. What does it mean to be creative? To me, it refers to the act of making something out of nothing or other things. So when one is creative in relationships, it can mean that one can form new things/situations, etc using relationships as a base ingredient. Which can include manipulation of people in some cases, though usually done in a most agreeable and charming manner.
A person with his/her non-Libran Sun in the 7th doesn’t necessarily have the above-mentioned creativity with people – unless there is a presence of Libran planets and/or angles in the chart. Someone with the Sun in the 7th probably suggest that his/her identity, life purpose, creativity, vitality, heroic journey, etc are most focused in the arena of relationships. The creativity employed in relationship depends on the sign. For example, if the Sun is in Aries, the creativity involved may be fearless confrontation, feel the fear and do it anyway. You can see that Sun (in Aries or other non-Libran sign) in the 7th lacks the creativity with people that Sun in Libra have.
To put it simply, a planet in a house is no substitute for a planet in a sign and vice-versa.