In case anyone is interested, here are the places along I35 between San Antonio and Waco where the frontage road does not go through. (Note, I didn't take southbound I35 from Waco to Salado so don't know about that stretch)
 
North of Kyle
Exits 215 to 217 - southbound only
Reason? unknown
 
Turn west at the Home Depot at Dry Hole Rd, which turns south. At the barricade either turn left on Old Bridge Trail then left into the shopping center parking lot then find your way back to the frontage road, or turn left and ride through the dirt path around the mud hole back to the frontage road.
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Georgetown  
Exits 259 to 261A - northbound only
Reason - routing through G'town was close enough?
 
Take S Austin Ave through Georgetown to exit 265. I suspect I'd route through G'town both ways as I think it's a little nicer than the frontage road.
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South of Belton  
Exits 289A to 289B - northbound only
Reason - Old Hwy 81 was close enough?
 
 Dumps you out on the I35 shoulder to cross the river.
 
Maybe take Toll Bridge Rd - Old US Hwy 81 at 289A back to I35 at exit 290 but you are only on I35 for about a block so it's probably not worth it.
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In Temple :
Exits 300 to 301 - both directions
Reason - train tracks
 
 We circled back to Ave H eastbound, left on 1741 (S 31st), left on W Adams back to the service road - didn't work well.
 
Exits 302 to 303A - both directions
Reason - train tracks
 
 We circled around to southbound frontage, turned right at Enterprise Rd, right at Eberhardt Rd to 363 (HK Dodgen Loop), back to I35 at exit 304. This one was a ways out of the way but not too bad.
 
It might work to take W Loop 363 around Temple. The northern end we took was very rideable but I don't know about the rest.
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North of Eddy  
Exits 318A to 318B - both directions
Reason - train tracks
 
 We circled around, right on First St, right on N Bruce, right on frontage road south, circle back under I35 to north frontage road. This was pretty workable and would be much easier when southbound as you don't have to cross under I35.
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Riding back I took 317 from Crawford to Belton. Decent chip-seal with a decent shoulder most of the way. South of Moody the shoulder narrows a bit but is still pretty easy. We heard that Spring Valley Rd was resurfaced with boulder-seal so we didn't attempt that route. In general, I'm still not sure how I would get around the Temple area if I needed to go through Waco. I'd probably try Loop 363 either to the east or the west. My sister is some new planner for traffic and other issues for Belton (she won't tell me her new job title or description :^) but she had recommended the loop to the east. It's further and she's not a cyclist so tends to be conservative. To the west might be fine if it's like the northern stretch we took. If you know, please report back.
 
On another note... I was stopped at the traffic light at W Central Eastbound and S 31st St and was intentionally right-hooked by a grey Dodge pickup. Knocked me off the bike and ran over my foot at the same time. Was a bit strange getting stretched out like that but it probably kept me from falling hard on my hip. No problem other than a sore left foot, however... I want to say that there were two motorcycle cops who came to the rescue almost before I had pulled myself up off the road. I'm used to all the crooked cops here in S.A. who hate cyclists, but these two guys completely amazed me with how professional, nice and helpful they were. I sure wish there were more out there like them. My sister knows the dispatchers for the county there and I asked her to try to find out who the two were and to thank them again for me if possible.

Wow Kent - it's great reading your ride reports, and setting PR's.  Do you have any upcoming races planned?   Have you considered riding solo at the TTTT later this fall?  Schlitter has the 24 hour and 500 mile bent record.  You should challenge his record.  Quite a few of the DFW area bent riders participate in this event.  I would be a great opportunity to integrate with us.  
http://www.tt24tt.com/