Nice sentiments, Graham, but I can't sense the timing of this.

The first line doesn't seem to fit the pattern.

There once was a man from Nantucket
Da-DA-da da-DA-da da-DA-da

Say if you took "I just had an incredible dream" and made it something like this:

I just had a wonderful dream

That fits.

And then there's supposed to be a second line of the same count, you see, for it to be a "limerick".

So maybe you could say:

I just had a wonderful dream
I saw OSensei by a stream

So there are your first two lines, fitting the meter pattern.

The next two would be paired, as well, but of a shorter meter:

da-DA-da da-DA
da-DA-da da-DA

You have:

O'Sensei said attack,
So I attacked where he sat,

You see, it misses the meter?

Perhaps if you said something like this:

He told me, "ATTACK!"
So I punched where he sat.

or "I kicked where he sat," or "I struck where he sat" or "I thrust" or whatever would fit that meter. You see?

And then the last line....

Found him gone yet I was where he'd been

which is ok but still a bit awkward.

How about: "He was gone, though I was where he'd been"?

And looking back, I guess you did have a first line. The way it appeared in the post, it looked like an opening comment, so I didn't realize it was part of the poem:

"May I just say, on a lighter theme"

Again, though, I think it's a bit loose on the meter:

Da-DA-da da-DA-da da-DA-da

But what if you said, "May I say, on a bit lighter theme...."

That fits.

So what if you phrased it like this:

May I say, on a bit lighter theme
that I just had a wonderful dream
O Sensei said "Attack!"
So I struck where he sat.
He was gone, though I was where he'd been.

But the meter...is still just a bit wobbly, isn't it? "O Sensei said 'attack'" just doesn't quite fit, does it?

So how about this:

I just had a wonderful dream.
I saw O Sensei by a stream.
He told me, "Attack!"
So I punched where he sat.
He was gone, though I was where he'd been.

It's rather like the footwork in aikido, isn't it? The timing has to be just so, or the technique is wobbly, or just falls apart.

What do you think?

Best wishes.

David