[Xy-pic] Change the orientation of Arrow of xtwocell

[Joe Hannon]

I'm trying to make a diagram similar to this one from wikipedia: (http://upload.wikimedia.org/wikipedia/commons/c/cb/Right_Kan_Extension.png)

Except that I want to the 2cell double arrow $\eta$ two be pointing from node $B$ in the bottom corner toward the middle of arrow $X$ in the top row(it is, after all, a natural transformation from $RF$ to $X$), whereas the diagram on wikipedia, which I was able to duplicate using the \xtwocell command, only wants to make the 2cell point in a direction perpendicular to some line chosen by the hop argument of the xtwocell command.

In subsection 3 (Excursions) of section 25.4 (extra options) of chapter 25 (two-cell) on page 47 of the xyrefer manual (http://www.ctex.org/documents/packages/graphics/xyrefer.pdf), it is stated 

 `\xtwocell[⟨hop⟩]{⟨displace⟩}...` connects to the ⟨pos⟩ displaced by ⟨displace⟩ from the relative cell location specified by ⟨hop⟩. The displacement can be any string of valid XY-pic commands, but they must be enclosed within a group {...}. When the cell location is the target, a null grouping {} must be given. When used with the <\omit> nudge, such excursions allow a labelled Arrow to be placed anywhere within an XY-pic diagram; furthermore the Arrow can be oriented to point in any direction.

So it sounds like it is possible to change the orientation of the arrow using the displace argument of the xtwocell command, but I can't figure out how to do so. What "string of valid XY-pic commands" can I use to change the orientation of the 2cell Arrow, if any?

Here's my example markup

\xymatrix{
A\ar[d]_F\ar[r]^X\xtwocell[1,0]{}\omit{^<-2>\eta} & C\\
B\ar@{-->}[ur]_R &
}

Nothing I tried in the displace argument of \xtwocell[1,0]{} changed the arrow in any way. 

Alternatively, I tried to use \..compositemap. This oriented the 2cell perfectly, but I could not figure out how to put a label at the common source/target of the composite arrows.

Any tips would be appreciated.

Regards,
Joe Hannon