I see. The charge does appear to be an alleged violation of 545.104 which states:
(a) An operator shall use the signal authorized by Section 545.106 (which authorizes hand signals) to indicate an intention to turn, change lanes, or start from a parked position.
(b) An operator intending to turn a vehicle right or left shall signal continuously for not less than the last 100 feet of movement of the vehicle before the turn.
(c) An operator may not light the signals on only one side of the vehicle on a parked or disabled vehicle or use the signals as a courtesy or "do pass" signal to the operator of another vehicle approaching from the rear.
I have attached a link to the Rules of the Road section of the Transportation Code here: http://www.statutes.legis.state.tx.us/Docs/TN/htm/TN.545.htm
I have underlined the subsection that applies to you, which is subsection (a). Subsection is not applicable to your case because it applies to a "turn" and you were not "turning" for purposes of the law.
So, assuming that this indeed is the violation you have been charged with (and, based on my review of the entire Transportation Code, this is the only law you could have been charged with violating), I would agree with you that the plain language of the law does not require you to maintain any signal throughout the entire lane change.
As for a Motion to Dismiss, this would not be the proper course to take. This is because Motions to Dismiss are only used in criminal matters where there is a facial defect in a pleading (i.e., the official charging document, which in this case would most likely be the actual ticket you received from the officer, was faulty). As an aside, the State will always have the opportunity to "amend" (or fix any error present in) the ticket.
However, because what appears to be the issue here would be the facts as they apply to your circumstance under the law, there must be a trial (if you choose to fight the ticket). This is because when there is a factual dispute (i.e., when you started the signal, when it ended, where you were in the lane when you began/ended signaling), a judge or jury has to determine which person's side of the story is more credible (your story or the officer's). Just because there is no video evidence of the portion of the lane change that is of issue does not preclude a prosecution of the offense. This is because the officer will simply testify to what he observed. (As you can imagine, there are hundreds of crimes a day not recorded by video that still lead to convictions because the officer testifies to his observations).
Of course, if there is no video of the exact issue that is the subject of the violation, this is advantageous to you. This is because it is the State that bears the burden of proving you guilty of the offense. The judge or jury can use the video at a trial to make reasonable deductions of the events that occurred (even if the portion of the lane change is not recorded). For instance, if there is something on the video that contradicts the officer's testimony (regarding what occurred during the unobstructed portion of the video), the court or jury may be more likely to disregard the officer's entire testimony (including the portion of the testimony averring to what occurred during the brief obstruction).
Please let me know if you have any follow up questions. I would be more than happy to answer them. If you do not have any other questions, please remember to click "accept" and leave positive feedback so I may receive credit for my response. Thank you!