Lots of variables affect the answer...and I'm not an engineer or home builder, but here's my amateur opinion.
The reason a hammock force is so high is b/c of a trig function...the combination of vertical and horizontal forces based on the angle of the hammock supports. If you're hanging from two ends of the same rafter, the horizontal force acts as compression to the rafter. Assuming you're not bowing
the rafter with the compression force, you're not going to compress a rafter to failure with hammock forces. So, if your rafter is one solid length all the way across (rather than two lengths of wood stuck together with a joiner), the only force you're really concerned with is the vertical forces. Depending on how it's constructed, your rafter may be able to handle that load. In theory.
But for stuff like this, do a risk analysis. Is the success worth the cost of possible failure? If this works, you get a new place to hang. If it fails, you could pull your roof down on top of you. Some rafters can handle it - there are even pics on the internet of folks hammocking from the rafters in AT shelters - but some rafters can't handle it. Look at your rafters...it's your call.
“Republics are created by the virtue, public spirit, and intelligence of the citizens. They fall when the wise are banished from the public councils because they dare to be honest, and the profligate are rewarded because they flatter the people, in order to betray them.” ~Judge Joseph Story
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IMPOSSIBLE JUST TAKES LONGER