By DIY Co2 system, do you mean yeast and sugar or do you have a compressed system with components you hand picked? Since it's 75 gals and you mentioned it was synched with a timer, I'll assume you are running pressurized and have a solenoid? If your yeast/sugar is synched to a timer, then I assume you mean you have your venturii pump plugged into the timer. Either way it doesn't really matter. If the power goes off, your Co2 either cuts out completely or stops being diffused and just bubbles out, depending on your setup. Either way, no power means no Co2. This shouldn't be a problem because when the power is out, so are your lights. No light means your plants aren't in high photosynthesis, so they don't need Co2. No lights means that algae shouldn't be actively growing either, so it's ok that your light and Co2 are low because you won't need the plants to out compete algae for nutrients. The Ph will likely climb as the Co2 outgasses to the atmosphere and reaches an equilibrium of about 3ppm in your water. This Ph swing shouldn't make much of a difference because it doesn't involve a change in Total Dissolved Solids, it's merely a function of acids and base buffers exchanging dominance. Your only concern would be that your plants could suffer if the power was out for many days and they got no light. They might grow a little leggy, but if the power is out long enough to kill them, then you have bigger problems on your hands, like trying to stay alive yourself. If it's winter, your heater would not have power and the temp would really drop unless you had a gas heater or something to keep the room warm.
My pressurized co2 system seems to handle thunderstorms well. The solenoid clicks off like every other electrical appliance, but unline my alarm clock, the solenoid goes right back to working condition once the power returns. As far as surges or electrical damage during a thunderstorm...I had to plug a powerstrip into my timer anyway to get two lights and solenoid running off it.
So really, dead of winter, no gas heater, power out for 6 days...you'd have a big problem, but it wouldn't be about your tank. Now, if your Co2 cut out and your tank lights somehow stayed on, then you could have an algae outbreak, or if your Kh was really low and your lights died but your Co2 stayed on, then you could possibly have dead fish problems if you were running very high Co2 levels and playing on the edge of disaster in the first place.
I don't see why it would be unsafe to add clean "same" substrate to your old tank. It might be inconvenient to have to uproot plants and have the water clouded, but as long as you are able to do a good water change afterwards to dilute any crap and excessive nutrients you disturbed up into the water column, it should't hurt anything. If I were willing to uproot my glosso, I would add another bag or two of flourite to mine.