(Note: I use the term 'intentionality' in reference to intention 1/7/13)
Andrew Lockley who runs geoengineeringinfo on twitter and is the moderator of the 'geoengineering discussion google group' tells me that regarding "sulphur in jet fuel" it "has already been considered and discarded by Antii - Ilari Partanen".
I looked it up, and found various interesting studies, here are two of them:
Stratospheric passanger flights are likely an inefficient geoengineering strategy
Antii - Ilari Partanen et al.
(Full free pdf) http://iopscience.iop.org/1748-9326/7/3/034021/pdf/1748-9326_7_3_034021.pdf
An another regarding ship sulphur content:
Climate and air quality trade-offs in altering ship fuel sulfur content
Antii - Ilari Partanen et al.
(Full free pdf) http://www.atmos-chem-phys.net/13/12059/2013/acp-13-12059-2013.pdf
I think that taken together, this two studies actually reinforce my view that 'we' already have been, and are geoengineering the climate. Even if in an 'unintentional', and given the accelerated rates of Arctic sea ice melt and ocean acidification , I would say; ineffective and perhaps even counter productive way. And that this, unintended geoengineering, has had and continues to have consequences beyond the 'wanted cooling effects' and the more immediate and well known unwanted pollution related deaths. With the negative consequences arguably been more heavily borne by the least responsible and least able to cope.
A couple of more points to ponder on:
-The 'confounding' 'warming hole' particularly over the central, eastern U.S. and Atlantic ocean   which are zones that seem to have heavier emissions from both aviation and shipping. 
This 'hiatus' seem to be readily cited by global warming skeptics, while downplaying the role of pollution.
- Do many of the proposed 'future' geoengineering start dates i.e 2020 and beyond have to do with scheduled (sulphur and other cooling pollution) reductions also from 2020 and beyond? 
Lastly, I would like to recall that famous phrase:
"Correlations does not imply causation"
(but it certainly should require investigation. O.E.)
"Correlation does not imply causation" is a phrase used in science and statistics to emphasize that correlation between two variables does not automatically imply that one causes the other (though correlation is necessary for linear causation, and can indicate possible causes or areas for further investigation... in other words, correlation can be a hint)" 
On this update two articles regarding the ETC group have been added to the 'other readings' section.
* Added on Jan. 8, 2014
**Added Feb 4, 2014 - in the ecuatorial zone and the lower stratophere in the northern hemisphere, also depending on the time of year**)
Alan Robock et al. - 10.1007/s10584-013-0777-5
Submited to Climatic Change November 2012
 Shipping pollution Emissions from shipping making ocean more acidic, researchers report
“Warming Hole” Over the Eastern U.S. Due to Air Pollution
 Atlantic heat sink explains global warming “pause”
 Lab releases global aviation emissions dataset
 A Year of Global Shipping Routes Mapped by GPS
 Impacts of emission reductions on aerosol radiative effects
"Correlation does not imply causation" (Princeton.edu)
*Plan to avert global warming by cooling planet artificially 'could cause climate chaos'
By Steve Connor - Science Editor - The Independent - January 8 2014
From Jan 12, 2015 update:
Forum Exchange – Is Climate Engineering Worthy of Consideration? – Andy Parker (IASS-Potsdam) and Jim Thomas (ETC Group)
Uncivil Debate (by ETC Group)