I think all your "rpm replicator" will need to output is a quasi-digital signal; something with a regular square wave. The 555 timer would be fine, and here is an alternative method.

I am reminded of my highschool electronics project in which I built an LED oscillator.

I found this schematic here
copy the ascii text to a fixed-width editor for the schematic. I could scan a better copy, but you would have to wait a while

All you need are:
- 2 NPN transistors (2n3904 or 2n2222 will do fine)
- 4 10kohm resistors
- 2 ~3uF capacitors
- One solder board from radioshack
- some wire

-----+--------+--------+--------+------> +12V
| | | |
R1 R2 R3 R4
| | | |
| + | | + +------> SIGNAL
+---C1---+ +---C2---+
c| | | |
\ | | |c
Q1 |------|--------+ b /
/ b +---------------| Q2
e| \
| |e
+--------------------------+------> GND

(note the polarity on the capacitors if you use electrolytic capacitors - the round cylinder ones)

Connect the circuit as shown, using the V+ and GND from the fan header

The operating frequency is dependent on the values of R2/R3 and C1/C3, approximately by the following formula:
f=3/(RC) - assuming R2/3 and C1/2 are respectively the same values.
Using the values above, you get 100Hz, or 6000rpm. You most likely won't find 3uF capacitors, so use the nearest standard values - 3.3uF or 2.2uF.

Hope this helps!