diodes to isolate pedal power branches

[R.G.]

So heres something I'm really curious about.   Are there any advantages (or disadvantages) to "piggybacking" an LM7809 off the output of an LM7818 that is "piggybacking" off the 24vdc module?

You're already thinking about the right things - total current and power/heating. One theoretical advantage is that any noise reduced by the first regulator will be reduced again by the second one. This is very much a theoretical advantage, as a single regulator is sufficient for every pedal I've seen.
So back at heating. A regulator wastes heat equal to the difference between its input voltage and its output voltage, times the actual output current. A single regulator might have too much input voltage to dissipate the heat it would otherwise have to. For instance, a 9V regulator providing 0.5A from an 18V supply much dissipate (18-9)*0.5 = 4.5W. That's over the 2W that a TO-220 package can dissipate without a heat sink, even if it's well within the 35-40dc that an LM7809 can stand. You can pay for a $0.50 to $2.00 heat sink, or put an LM7815 between the 18V and the 9V regulator. The 7815 now dissipates 3V*0.5A = 1.5W, and the 7809 dissipates 6V*0.5A = 3W. Notice it's still 4.5W, But you've moved some of the heat to the 7815 and out of the 7809. This can be carried on: a 7812 between the 7815 and 7809 gets it down to 3V*0.5A=1.5W for all of them.
And it will confuse the dickens out of anyone trying to copy your circuits.   8-)

[pd3]

Thank you for that feedback R.G.   

And that will likely be sufficient info to do exactly that.   Having that I can really use a big long/wide aluminum box to mount regulators, while even adding extra normal heatsinks if I'd like to be silly about it, I'd suspect the heat will be spread out enough to safely diffuse without issue.  Another boon is that there likely will not be alot of current being drawn out of the stompbox's I intend to power - (160mA for a Keeley Halo, around 30mA for a Keeley MK3 driver and, well, I'll need to check the current for a particual MIAB type pedal.  The issue may come when its time to employ those nice expensive digital reverbs.   Hey, if Its better to just have separate PS's for those, then so be it.  Its looking good for those smaller pedals for now.  The Keeley MK3 driver is said to have some extra sauce (dynamic range) at 18vdc so, whynot!   

Thank you all.  Regs come in from Digikey tomorrow and I'll set up a out-of-box ciruit to look at function, noise, maybe ripple on my scope.  Cool !  Going to try to make this work,

thank you all!
Best,
PD3

[ElectricDruid]

If you're going to bolt the regulators to the box, make sure that all the tabs have the same potential. The tab on a TO220 package regulator is electrically connected to the internals, but I can't remember to what. I do remember having to add silica washers and sleeves to prevent shorting on some power supply designs. It might have been bipolar though, mixing 78xx and 79xx, so this is just a heads-up for something to watch out for.

[PRR]

I can't remember to what

Traditionally, "the most negative internal node". A chip is full of parasitic diodes, this keeps them all wrong-way, inactive. 7805, tab is GND. 7905, tab is IN.

BUT the "Adj" parts have a isolation moat so the "ADJ" pin is not the tab. On the Positive version, tab is OUT (and Vadj sits 1.2V more negative). On the Negative version, memory fails, go to the manual.

About the only time I've not isolated was for the gas-gauge in a 1972 mail-truck (most 12V vehicles). These used a 6V gauge (why re-tool?) with a blinker-like voltage dropper (starts off more accurate). When this gets old the gauge reads way-out. I had a plan to mount a 7805 on the gauge's terminal stud (yes, you can drill-out a TO220 tab). Reads just a hair low but very stable against battery and heat variation.

[mozz]

TO-220F or FP have isolated tabs.

Edit: NJM7809FA $1.28 at Mouser. 

[PRR]

TO-220F or FP have isolated tabs.

That's so! They seem "new" to me but they aren't.

[antonis]

TO-220F or FP have isolated tabs.

With over-double thermal resistance..

[Rob Strand]

That's so! They seem "new" to me but they aren't

I don't recall seeing  the common parts like LM7805 in TO-220FP either but I have seen plenty of other regulators in TO-220FP.    They are very common in consumer equipment since you don't need washers, insulator.   (Transistors are also available.)

This one is pretty close:
https://www.st.com/resource/en/datasheet/l78.pdf

Thermal resistances vary a lot from model to model.  You really need to check the specific datasheet.