"jcatora"
I'm a Registered Architect, so let's see if I can lend a little clarity
here...

Terminology:
If your house was 'conventionally framed' (that is, the old 'stick'
method), the sloped members that support the roof deck would be called
"RAFTERS" and the horizontal members that are supporting your ceiling
are called "CEILING JOISTS".

If (as it appears) your roof structure is framed with wood trusses,
these members are called "CHORDS" - the 'top chord' being that which
support the roof deck and the 'bottom chord' being that which supports
the ceiling.

As you've described this as '2x4' framing, it sounds pretty clear this
is, indeed, 'trusses' - as a normal ceiling joist would be quite a bit
bigger (like 2x8 or so).

The important point about 'trusses' is that they are designed and
assumed to carry their load at the 'panel points' (that point where
some of the diagonal members frame into either the top or bottom chord
(usually with those little 'prickly' metal plates on each side). Add to
this the fact that a truss will resolve the forces into axial ones
(i.e. running down the length of the member) and this is why the chords
of trusses are smaller - they're not 'bending' as much (other than
carrying the actual load of the ceiling or roof decking) -- a truss is
just simply more efficient in doing this (that's why they're used).

Another important point to consider when adding some additional load to
a truss is to get it as close to one of the 'panel points' as possible
- that way, you are not loading the chords in 'bending' (which they
were not assumed to be doing initially). A 200 or 400 pound load is
not that much when carried at one of these 'panel points' - but might
get 'iffy' if you applied this load midway between two 'panel points'.

Your idea of laying a 2x6 across 3 (or so) truss chords is a good one -
but, to improve it a little more, I would suggest making a 'strongback'
- and, by that, I mean laying your 2x6 horizontal across the 3 bottom
chords but also nailing (at least) a 2x4 VERTICAL to one side of this
2x6 (better yet - use a 2x6 vertical here, as well). It is difficult
to put a bolt through the long axis of a piece of lumber - better to
go through the short dimension (like drilling through that flat 2x6) -
but this is also the 'weak axis' of the member (the "Moment of Inertia"
mentioned earlier) - which is why I suggest turning (and nailing) that
vertical 2x6 to it -- making it a 'strongback'. Nail these together
with 16d nails about every 8 to 12" (no more) and use machine thread
eye-bolts, not the 'lag thread'. One thing I didn't see mentioned is
the fact of the eye-bolt opening up under load -- you can get forged
eye-bolts (i.e. with a 'closed eye') that is 10 times (or more) stonger
that would give you a lot of insurance at only minor cost increase.
Lowe's (etc) may have it - or McMaster-Carr online surely does. I
might would suggest using those forged ones -- because they are (truly)
at least 10 times stronger and if this is a 'human' hanging off of
these eyebolts, that about cinches the decision (in my mind).

Main points:
Put your load as close to a 'panel point' as possible
Add that vertical 'strongback' to the side of your framing member
Use forged (closed eye) eyebolts
Be careful and look for signs of fatigue when loading it - wood
won't fail immediately, it gives warnings (assuming you're paying
attention).

-- john.


Finally, an accurate, well thought out response.

Thanks

Dave