A couple of tweets with pictures of sign pr0n:
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What I can't understand is where they stuck the sign; it appears to be in the median of the future main lanes west of I-35, which probably is a good place for a photo op but you'd never see it from the road.
Luckily I noticed this thread on my way to Texas, so I got to Laredo a few days after the ribbon-cutting and could add Loop 20 to today's itinerary. Timing is everything, huh?
EDIT: The Future I-69 Corridor sign shown in the above tweets is on eastbound Loop 20 west of I-35, just east of where the main lanes end and split into two off-ramps. The sign is placed in the median, to the left of the exit ramp onto the frontage road for traffic continuing on Loop 20 or to SB I-35, and to the right of where the main lanes go once they're extended. That's the only Future I-69 Corridor sign posted anywhere on Loop 20 in either direction.
This photo might help a little. I took the photo from the median between the frontage road and the two off-ramps noted above. The flyover in the background is for traffic to WB Loop 20, from both SB and NB I-35.

A few other notes (not reflecting all the discussion upthread, I'm just laying out my observations even if some are not new news):
Loop 20 west of I-35 is basically a full freeway to the commercial traffic-only World Trade Bridge border crossing over to Nuevo Laredo. There is an incomplete set of flyover ramps (SB 35 to WB 20, NB 35 to WB 20, EB 20 to NB 35 -- there's a stub end for an unbuilt connector for EB 20 to SB 35), providing freeway-to-freeway connections. The only exit west of I-35, and the last one until the border, is with FM 1472.
There's a minor at-grade intersection just short of the border, allowing some EB traffic to access some export facility right after clearing customs, and WB traffic a last chance to turn back before the border (which I did). Here's two photos of WB Loop 20 at the closed bridge entrance (the bridge has limited hours, especially on Sundays, but it was open the first time I stopped by that day), showing the intersection and details of the signs there:


The rest of Loop 20 is not yet even close to Interstate-grade. EB Loop 20 traffic continuing east of I-35, and WB traffic heading west of i-35, has to go onto frontage roads, through two signalized intersections with I-35 ramps. There are more to the east of the I-35 ramps. Loop 20 between I-35 and US 59 is a mix of frontage roads with a wide median roomy enough to handle a freeway conversion (but only one overpass in the median), and divided or undivided highway with no obvious provisions to upgrade to freeway. So it doesn't surprise me that there's no Future I-69 corridor signage east of I-35.
Nor is there any signage indicating that Loop 20 includes the future re-routing of US 59, which for now continues into downtown Laredo as before.