Suggestions for a passive line-level high-pass crossover?

Do a vertical biamp instead. You won't need the xover.

" Bi-amping vertically (rather than horizontally) does not negate the need for a x/o to accomplish. "

Unless you have some really odd equipment, that's not true. There's no xovers on my vertical biamp system, or any other one I know of.

" Neither of the two amps in a bi-amped system knows whether it is being used vertically or horizontally (other than in the sense of how the amp and speaker interact with each other), and that consideration has nothing to do with the x/o itself. "

The amp may not know, but you need to.

" Using a solid state amp for the low-pass (bass) signal on both speakers, and a tube amp for the high-pass (mids and highs) on both, works great with many speakers."

You'll get sound, but its usually an unbalanced train wreck. When you put an active xover in the system, you can get the levels right, but now you have to listen to one amp on the mids and highs, and another on the lows. Who would want that? Do it right the first time and be done with it. Also, don't forget that the active xo will have a big impact on SQ. It will negate a good portion of what your source, cables and preamp are doing.

" The lucky owners of Vandersteen 7’s, for one. That speaker comes with a powered (via a solid state amp) subwoofer, and many 7 owners use a tube power amp for the upper drivers. People have done that successfully for decades, with speakers like the Infinity RS-1b, for one. "

I thought it would be self evident that I was referring high's, mid's and low's, and not sub frequencies.  I'm one of the people you are referring to that's been doing it for decades. I did it with Vandersteen, Genesis, Martin Logan, B&W, Unity, Audio Physic and probably a few others that I'm forgetting. 

" sfall, if you don’t have an external x/o splitting the signal into two or three sub-signals, you are sending a full-range signal to all the drivers---not a good idea! "

That's just not true. The only way that would happen is to bypass the internal xover.

" Are you sure you’re not just using a pair of stereo amps to in effect bi-wire your speakers?"

Yes. The only way you can biwire a speaker is to use speaker cables. That's why they call it biwiring.

" Bi-amping absolutely requires an external x/o, whether passive, active, or a combination of both. If an external x/o (external in the sense of not inside the speaker. A passive filter can be inside the power amp, on it’s input jacks) is not present, bi-amping is not, by definition, being accomplished. "

That's completely false as well. For starters, you can look at my system. No expernal xover to be found. I don't know where you are getting these definitions from, but you may want to find a new source.

" The term “vertical biamping” was coined to describe a biamp configuration where one channel of a stereo amplifier drives the bass section of a single loudspeaker and the other channel drives the treble section of the same speaker, as opposed to the commercial biamp configuration where one channel of a stereo amplifier would drive the bass section of the left speaker and the other channel would drive the bass section of the right speaker, and another amplifier would drive the treble sections of these same speakers.

Passive biamplification means that an electronic crossover is not used and the signal is divided into frequency bands by the passive crossover within the speaker. This allows the passive crossover network to perform its job of equalizing the response of individual drive elements and correcting for phase anomalies.

One channel of the left stereo amp drives the bass section of the left speaker and the other channel of the same amplifier drives the treble section of the left speaker. One channel of the right stereo amp drives the bass section of the right speaker and the other channel drives the treble section of that speaker.No external crossover is required and the internal crossover network in each speaker is still utilized. The advantages are many.The heavy current demands of the bass sections of each speaker can now be shared by two amplifiers and two power supplies. Each midrange/tweeter section will now have a dedicated amplifier channel. This will reduce cross talk for better imaging and provide a substantial increase in dynamic range while minimizing the potential for speaker damage caused biamplifier clipping. If the bass amplifier clips it won’t deliver high energy, high frequency distortion to the tweeters—the primary cause of speaker failure." 

APJ 4 P.16

It's pretty clear, but if you have info that says otherwise, I'll certainly look at it.