Simon Mann and 69 others - all travelling on South African passports - were arrested in Harare on 7 March, allegedly on their way to stage the coup.
Another 15 foreigners, including eight South Africans, were detained on similar charges in Malabo, the capital of the Central African nation.
Sentencing is due to begin on 10 September.
An alleged desperate written appeal for help that Mann managed to smuggle out of his high-security prison in Zimbabwe drew attention to Mark Thatcher, the son of former British prime minister Margaret Thatcher.
Thatcher was arrested in Cape Town on Wednesday and charged with bankrolling the alleged putsch.
His arrest is shining a spotlight on powerful members of the British elite who may have colluded to take over the oil-rich African country.
Other names from rarefied circles have been linked to the conspiracy, including disgraced writer Lord Jeffrey Archer, London-based businessman Ely Cahlil and millionaire David Hart, who is said to be close to Lady Thatcher.
Mark Thatcher, 51, has denied the charge of putting up $275,000 to fund the coup attempt against Equatorial Guinea President Teodoro Obiang Nguema.
Since 1979, Nguema has been at the helm of the country where huge oil reserves were discovered recently, attracting the attention of several foreign firms and governments.
Mark Thatcher (L) has denied that
he funded the coup attempt
Mark Thatcher's friendship with Mann, one of the alleged masterminds, who was also educated in private British schools and the scion of a Watney brewing empire, highlighted the "old boy" network of the alleged soldiers of fortune and their patrons.
Mann's letter, replete with public-school jargon, said all those involved including "Scratcher" - a presumed reference to Thatcher - should pitch in and help, according to media reports.
Mann reportedly said bribes could settle the affair, evoking the possible need for a "large splodge of wonga", adding: "Now it's bad times and everyone has to pull their ... weight."
Mann and Nick du Toit, a South African on trial as a co-conspirator in Equatorial Guinea, set up Executive Outcomes, which operated from Pretoria in South Africa and helped the Angolan government protect its oil installations from rebels during the war.
No special treatment
South Africa's This Day daily, in a cutting editorial on Thursday, highlighted the privileged backgrounds of the alleged key players, describing Thatcher as an "old Harrovian" and Mann as an "old Etonian well connected to the Conservative party".
It pointed out that while mercenaries had a "long and dishonourable history of plying their trade in Africa ... a particular kind ... the colonially-minded British expat has come to typify this species".
Mercenaries had a "long and dishonourable history of plying their trade in Africa ... a particular kind ... the colonially-minded British expat has come to typify this species"
This Day, South African daily
A spokesman for South Africa's elite Scorpions investigating unit, Makhosini Nkosi, has said Thatcher will not be accorded any special privileges despite being the offspring of the formidable "Iron Lady".
"Mr Thatcher is an ordinary resident of South Africa. He is not royalty nor is he an ex-head of state," he said.