Easy way is separate boost converters with N:1 transformers.
For super simple a 1:1 choke used as a transformer and driven with a transistor rate to 100V+ will suffice.
N channels can be driven in parallel.
Choke primary in say MOSFET drain to V+
Pulse gate on, turn off.
Drain rings to 100V+ and is rectified to output cap.
100V x 10 mA = 1 Watt/channel = non trivial.
On period designed to just start to saturate inductor core, or less.
Repetition rate & thus mark space ratio related to power required.
Output regulation can be by zener per output.
If you want N x stand alone IC's my favourite olde school MC34063 + external transistor to cover voltage is cheap and flexible.
The MC34063 datasheet doesn't seem to mention isolated supplies.
Can it do them?
Yes. This is essentially doing what the FET based cct that I discuss in more detail below does BUT using the IC to look after the control. Add secondary winding to inductor. Note that this MAY need to be modified very slightly so a small capacitor is charged on input side to provide a sense voltage for the feedback BUT it can also be run open loop with a little playing. For the general idea see ...
fig9a here with a winding added to the inductor
Pin 1 would not want to be returned to the inductor as shown as it sees full flyback voltage.
Added - comment on added circuit in question.
Your circuit (almost) shows the pulsed inductor approach I mentioned.
An N:1 inductor pair is "nice" BUT 1:1 pairs are available off the shelf premade and very cheap as differential or longitudinal noise chokes (how the dotting is used affects which they affect). These are wound on largish ferrite beads. You want one with enough inductance and current capability when one winding is driven with DC and enough frequency response and low enough loss (draws deep breath) that it can be used here. They exist.
I_in_peak ~= Vin x t_on / L
Energy in inductor = 0.5 x L x i_in^2
Energy out = Energy_in_inductor x f
So power ~= <= :-) 0.5 x L x (Vin x ton / L)^2 x f
= 0.5 x (Vin x ton)^2 /L x f -*
( = 0.5 x L x i^2 x f )
Off time is small as 1:1 inductor will discharge in ~~= Vin/Vout x ton
So energy transfer can be controlled by on pulse rate.
Max on time is set by inductor ability to handle Iin.
In this case D1 is not required and would prevent operation.
C & Z now act as the clamp.
And the output side needs only a single diode Dout from the non-dotted side of the secondary to Vout+ with the dotted side going to Dout-.
When Q is on you get small negative output which is blocked by Dout. When Q is turned off Q_drain rings positive so Lout undotted rises to as high as is needed to allow magnetic flux to vary smoothly continuous. Dout will conduct and clamp the voltage at Lin to the same as Vout - ie FET will see say 100 V at drain. In a lossless system the output will deliver N times the voltage for 1/Nth of the on time.
-* E&OE - Rushing, written on the fly unchecked, should be right, may be wrong due to typo, idea is OK.